### 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 aliases is None:
        aliases = _handle_aliases(argument_spec, parameters)
    return list(aliases.keys()) + list(argument_spec.keys())
def _get_unsupported_parameters(argument_spec, parameters, legal_inputs=None, options_context=None):
    """Check keys in parameters against those provided in legal_inputs
    to ensure they contain legal values. If legal_inputs are not supplied,
    they will be generated using the argument_spec.
    :arg argument_spec: Dictionary of parameters, their type, and valid values.
    :arg parameters: Dictionary of parameters.
    :arg legal_inputs: List of valid key names property names. Overrides values
        in argument_spec.
    :arg options_context: List of parent keys for tracking the context of where
        a parameter is defined.
    :returns: Set of unsupported parameters. Empty set if no unsupported parameters
        are found.
    """
    if legal_inputs is None:
        legal_inputs = _get_legal_inputs(argument_spec, parameters)
    unsupported_parameters = set()
    for k in parameters.keys():
        if k not in legal_inputs:
            context = k
            if options_context:
                context = tuple(options_context + [k])
            unsupported_parameters.add(context)
    return unsupported_parameters
def _handle_aliases(argument_spec, parameters, alias_warnings=None, alias_deprecations=None):