### 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
if isinstance(sub_param, string_types):
                            sub_param = check_type_dict(sub_param)
                        if not isinstance(sub_param, Mapping):
                            raise TypeError("Value '{1}' in the sub parameter field '{0}' must by a {2}, "
                                            "not '{1.__class__.__name__}'".format(arg_name, sub_param, wanted_type))
                        no_log_values.update(_list_no_log_values(sub_argument_spec, sub_param))
    return no_log_values
def _return_datastructure_name(obj):
    """ Return native stringified values from datastructures.
    For use with removing sensitive values pre-jsonification."""
    if isinstance(obj, (text_type, binary_type)):
        if obj:
            yield to_native(obj, errors='surrogate_or_strict')
        return
    elif isinstance(obj, Mapping):
        for element in obj.items():
            for subelement in _return_datastructure_name(element[1]):
                yield subelement
    elif is_iterable(obj):
        for element in obj:
            for subelement in _return_datastructure_name(element):
                yield subelement
    elif isinstance(obj, (bool, NoneType)):
        
        return
    elif isinstance(obj, tuple(list(integer_types) + [float])):
        yield to_native(obj, nonstring='simplerepr')
    else:
        raise TypeError('Unknown parameter type: %s' % (type(obj)))
def _remove_values_conditions(value, no_log_strings, deferred_removals):