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.