### 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
"""Return set of no log values
    :arg argument_spec: An argument spec dictionary
    :arg params: Dictionary of all parameters
    :returns: :class:`set` of strings that should be hidden from output:
    """
    no_log_values = set()
    for arg_name, arg_opts in argument_spec.items():
        if arg_opts.get('no_log', False):
            
            no_log_object = params.get(arg_name, None)
            if no_log_object:
                try:
                    no_log_values.update(_return_datastructure_name(no_log_object))
                except TypeError as e:
                    raise TypeError('Failed to convert "%s": %s' % (arg_name, to_native(e)))
        
        sub_argument_spec = arg_opts.get('options')
        if sub_argument_spec is not None:
            wanted_type = arg_opts.get('type')
            sub_parameters = params.get(arg_name)
            if sub_parameters is not None:
                if wanted_type == 'dict' or (wanted_type == 'list' and arg_opts.get('elements', '') == 'dict'):
                    
                    if not isinstance(sub_parameters, list):
                        sub_parameters = [sub_parameters]
                    for sub_param in sub_parameters: