### 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
'diff': ('_diff', False),
    'keep_remote_files': ('_keep_remote_files', False),
    'module_name': ('_name', None),
    'no_log': ('no_log', False),
    'remote_tmp': ('_remote_tmp', None),
    'selinux_special_fs': ('_selinux_special_fs', ['fuse', 'nfs', 'vboxsf', 'ramfs', '9p', 'vfat']),
    'shell_executable': ('_shell', '/bin/sh'),
    'socket': ('_socket_path', None),
    'string_conversion_action': ('_string_conversion_action', 'warn'),
    'syslog_facility': ('_syslog_facility', 'INFO'),
    'tmpdir': ('_tmpdir', None),
    'verbosity': ('_verbosity', 0),
    'version': ('ansible_version', '0.0'),
}
PASS_BOOLS = ('check_mode', 'debug', 'diff', 'keep_remote_files', 'no_log')
DEFAULT_TYPE_VALIDATORS = {
    'str': check_type_str,
    'list': check_type_list,
    'dict': check_type_dict,
    'bool': check_type_bool,
    'int': check_type_int,
    'float': check_type_float,
    'path': check_type_path,
    'raw': check_type_raw,
    'jsonarg': check_type_jsonarg,
    'json': check_type_jsonarg,
    'bytes': check_type_bytes,
    'bits': check_type_bits,
}
def _get_type_validator(wanted):