### 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
"""
    Helper function for :meth:`remove_values`.
    :arg value: The value to check for strings that need to be stripped
    :arg no_log_strings: set of strings which must be stripped out of any values
    :arg deferred_removals: List which holds information about nested
        containers that have to be iterated for removals.  It is passed into
        this function so that more entries can be added to it if value is
        a container type.  The format of each entry is a 2-tuple where the first
        element is the ``value`` parameter and the second value is a new
        container to copy the elements of ``value`` into once iterated.
    :returns: if ``value`` is a scalar, returns ``value`` with two exceptions:
        1. :class:`~datetime.datetime` objects which are changed into a string representation.
        2. objects which are in ``no_log_strings`` are replaced with a placeholder
           so that no sensitive data is leaked.
        If ``value`` is a container type, returns a new empty container.
    ``deferred_removals`` is added to as a side-effect of this function.
    .. warning:: It is up to the caller to make sure the order in which value
        is passed in is correct.  For instance, higher level containers need