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.