### 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
for arg_name, arg_opts in argument_spec.items():
        if arg_name in parameters:
            if prefix:
                sub_prefix = '%s["%s"]' % (prefix, arg_name)
            else:
                sub_prefix = arg_name
            if arg_opts.get('removed_at_date') is not None:
                deprecations.append({
                    'msg': "Param '%s' is deprecated. See the module docs for more information" % sub_prefix,
                    'date': arg_opts.get('removed_at_date'),
                    'collection_name': arg_opts.get('removed_from_collection'),
                })
            elif arg_opts.get('removed_in_version') is not None:
                deprecations.append({
                    'msg': "Param '%s' is deprecated. See the module docs for more information" % sub_prefix,
                    'version': arg_opts.get('removed_in_version'),
                    'collection_name': arg_opts.get('removed_from_collection'),
                })
            
            sub_argument_spec = arg_opts.get('options')
            if sub_argument_spec is not None:
                sub_arguments = parameters[arg_name]
                if isinstance(sub_arguments, Mapping):
                    sub_arguments = [sub_arguments]
                if isinstance(sub_arguments, list):
                    for sub_params in sub_arguments:
                        if isinstance(sub_params, Mapping):
                            deprecations.extend(_list_deprecations(sub_argument_spec, sub_params, prefix=sub_prefix))
    return deprecations
def _list_no_log_values(argument_spec, params):