### 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
continue
        wanted_type = spec.get('type')
        type_checker, wanted_name = _get_type_validator(wanted_type)
        
        
        kwargs = {}
        if wanted_name == 'str' and isinstance(wanted_type, string_types):
            kwargs['param'] = list(parameters.keys())[0]
            
            if prefix:
                kwargs['prefix'] = prefix
        try:
            parameters[param] = type_checker(value, **kwargs)
            elements_wanted_type = spec.get('elements', None)
            if elements_wanted_type:
                elements = parameters[param]
                if wanted_type != 'list' or not isinstance(elements, list):
                    msg = "Invalid type %s for option '%s'" % (wanted_name, elements)
                    if options_context:
                        msg += " found in '%s'." % " -> ".join(options_context)
                    msg += ", elements value check is supported only with 'list' type"
                    errors.append(ArgumentTypeError(msg))
                parameters[param] = _validate_elements(elements_wanted_type, param, elements, options_context, errors)
        except (TypeError, ValueError) as e:
            msg = "argument '%s' is of type %s" % (param, type(value))
            if options_context:
                msg += " found in '%s'." % " -> ".join(options_context)
            msg += " and we were unable to convert to %s: %s" % (wanted_name, to_native(e))
            errors.append(ArgumentTypeError(msg))
def _validate_argument_values(argument_spec, parameters, options_context=None, errors=None):