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

Commit 7a483656 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.

Cc: Dan Carpenter <dan.carpenter@oracle.com>
Reported-by: kernel test robot <lkp@intel.com>
Fixes: 7a483656 ("wifi: wil6210: debugfs: fix info leak in wil_write_file_wmi()")
Fixes: ff974e40 ("wil6210: debugfs interface to send raw WMI command")
Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20220724202452.61846-1-ammar.faizi@intel.com