### Summary

An exception is thrown when an argspec expecting a dict with suboptions is given a string.
This only occurs when the argument in the argspec has suboptions.

### Issue Type

Bug Report

### Component Name

module_utils/common/parameters.py

### Ansible Version

```console
ansible [core 2.11.4]
```


### Configuration

```console
$ ansible-config dump --only-changed
```


### OS / Environment

RHEL 8

### Steps to Reproduce

`meta/argument_specs.yml`
```yaml
---
argument_specs:
  main:
    options:
      dict_option:
        type: dict
        options:
          some_option:
            type: str
```
Playbook:
```
---
- hosts: localhost
  gather_facts: false
  vars:
    dict_option: this is a string
  roles:
    - test
```

### Expected Results

An exception should not be thrown.

### Actual Results

```console
The full traceback is:
Traceback (most recent call last):
  File "/home/user/ansible4/lib/python3.8/site-packages/ansible/executor/task_executor.py", line 158, in run
    res = self._execute()
  File "/home/user/ansible4/lib/python3.8/site-packages/ansible/executor/task_executor.py", line 582, in _execute
    result = self._handler.run(task_vars=variables)
  File "/home/user/ansible4/lib/python3.8/site-packages/ansible/plugins/action/validate_argument_spec.py", line 83, in run
    validation_result = validator.validate(provided_arguments)
  File "/home/user/ansible4/lib/python3.8/site-packages/ansible/module_utils/common/arg_spec.py", line 244, in validate
    _validate_sub_spec(self.argument_spec, result._validated_parameters,
  File "/home/user/ansible4/lib/python3.8/site-packages/ansible/module_utils/common/parameters.py", line 761, in _validate_sub_spec
    unsupported_parameters.update(_get_unsupported_parameters(sub_spec, sub_parameters, legal_inputs, options_context))
  File "/home/user/ansible4/lib/python3.8/site-packages/ansible/module_utils/common/parameters.py", line 177, in _get_unsupported_parameters
    for k in parameters.keys():
AttributeError: 'str' object has no attribute 'keys'
fatal: [localhost]: FAILED! => {
    "msg": "Unexpected failure during module execution.",
    "stdout": ""
}
```


### Code of Conduct

- [X] I agree to follow the Ansible Code of Conduct
"""Sanitize the keys in a container object by removing ``no_log`` values from key names.
    This is a companion function to the :func:`remove_values` function. Similar to that function,
    we make use of ``deferred_removals`` to avoid hitting maximum recursion depth in cases of
    large data structures.
    :arg obj: The container object to sanitize. Non-container objects are returned unmodified.
    :arg no_log_strings: A set of string values we do not want logged.
    :kwarg ignore_keys: A set of string values of keys to not sanitize.
    :returns: An object with sanitized keys.
    """
    deferred_removals = deque()
    no_log_strings = [to_native(s, errors='surrogate_or_strict') for s in no_log_strings]
    new_value = _sanitize_keys_conditions(obj, no_log_strings, ignore_keys, deferred_removals)
    while deferred_removals:
        old_data, new_data = deferred_removals.popleft()
        if isinstance(new_data, Mapping):
            for old_key, old_elem in old_data.items():
                if old_key in ignore_keys or old_key.startswith('_ansible'):
                    new_data[old_key] = _sanitize_keys_conditions(old_elem, no_log_strings, ignore_keys, deferred_removals)
                else:
                    
                    
                    new_key = _remove_values_conditions(old_key, no_log_strings, None)
                    new_data[new_key] = _sanitize_keys_conditions(old_elem, no_log_strings, ignore_keys, deferred_removals)
        else:
            for elem in old_data:
                new_elem = _sanitize_keys_conditions(elem, no_log_strings, ignore_keys, deferred_removals)
                if isinstance(new_data, MutableSequence):
                    new_data.append(new_elem)
                elif isinstance(new_data, MutableSet):
                    new_data.add(new_elem)