### 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 value.get('apply_defaults', False):
                if sub_spec is not None:
                    if parameters.get(param) is None:
                        parameters[param] = {}
                else:
                    continue
            elif sub_spec is None or param not in parameters or parameters[param] is None:
                continue
            
            options_context.append(param)
            
            if isinstance(parameters[param], dict):
                elements = [parameters[param]]
            else:
                elements = parameters[param]
            for idx, sub_parameters in enumerate(elements):
                if not isinstance(sub_parameters, dict):
                    errors.append(SubParameterTypeError("value of '%s' must be of type dict or list of dicts" % param))
                
                new_prefix = prefix + param
                if wanted == 'list':
                    new_prefix += '[%d]' % idx
                new_prefix += '.'
                no_log_values.update(set_fallbacks(sub_spec, sub_parameters))
                alias_warnings = []
                alias_deprecations = []
                try:
                    options_aliases = _handle_aliases(sub_spec, sub_parameters, alias_warnings, alias_deprecations)
                except (TypeError, ValueError) as e:
                    options_aliases = {}