wifi: wil6210: debugfs: fix uninitialized variable use in `wil_write_file_wmi()`

In the Linux kernel, the following vulnerability has been resolved: wifi: wil6210: debugfs: fix uninitialized variable use in `wil_write_file_wmi()` Commit 7a4836560a61 changes simple_write_to_buffer() with memdup_user() but it forgets to change the value to be returned that came from simple_write_to_buffer() call. It results in the following warning: warning: variable 'rc' is uninitialized when used here [-Wuninitialized] return rc; ^~ Remove rc variable and just return the passed in length if the memdup_user() succeeds.