### Step 1: Root Cause Analysis

#### Understanding the Code:
1. **Function Overview**:
   - The function `mwifiex_cfg80211_start_ap` is responsible for starting an access point (AP) with given settings.
   - It performs various checks and configurations, allocates memory for `bss_cfg`, and sets up the AP parameters.

2. **Memory Allocation**:
   - At line 13, memory is allocated for `bss_cfg` using `kzalloc`.
   - If the allocation fails, the function returns `-ENOMEM` (lines 14-17).

3. **Parameter Checks and Configuration**:
   - The function sets various parameters in `bss_cfg` based on `params`.
   - It handles the `hidden_ssid` parameter with a switch-case statement (lines 32-42).

4. **Potential Vulnerability**:
   - If the `hidden_ssid` parameter does not match any of the expected cases, the default case is executed (line 41).
   - The default case returns `-EINVAL` (line 42) without freeing the allocated memory for `bss_cfg`.

#### Identifying the Vulnerability:
- **CWE-401: Memory Leak**:
  - The memory allocated for `bss_cfg` is not freed if the function returns `-EINVAL` in the default case of the switch statement.
  - This results in a memory leak, as the allocated memory is not released back to the system.

### Step 2: Fixing Strategy

#### Ensuring Proper Memory Deallocation:
- To fix the memory leak, we need to ensure that `bss_cfg` is freed before returning `-EINVAL` in the default case of the switch statement.

#### Applying the Fix:
- Modify the default case to include a call to `kfree(bss_cfg)` before returning `-EINVAL`.

### Conclusion:
The patch ensures that the allocated memory for `bss_cfg` is properly freed in the default case of the switch statement, preventing a memory leak.

### Patch:
```diff
<     default:
<         return -EINVAL;
---
>     default:
>         kfree(bss_cfg);
>         return -EINVAL;
```

### Reasoning Steps for Generating the Patch:
1. **Identify the Memory Allocation**:
   - Recognize that `bss_cfg` is allocated memory using `kzalloc`.

2. **Locate the Vulnerable Code**:
   - Identify the default case in the switch statement where the function returns `-EINVAL` without freeing `bss_cfg`.

3. **Determine the Fix**:
   - Ensure that `bss_cfg` is freed before returning from the function in the default case.

4. **Apply the Fix**:
   - Add a call to `kfree(bss_cfg)` before the return statement in the default case.

By following these steps, the patch effectively addresses the CWE-401 vulnerability by ensuring proper memory deallocation.