rtw_enqueue_cmd() uses a semaphore to notify rtw_cmd_thread() that it
has enqueued commands. rtw_cmd_thread() "down(s)" in interruptible mode
to wait to be notified.
Use completion variables because they are better suited for the purpose.
In rtw_cmd_thread(), wait in uninterruptible mode, even if the original
code uses down_interruptible(), because the interruption of
rtw_cmd_thread() is not allowed and unwanted.
Tested with "ASUSTek Computer, Inc. Realtek 8188EUS [USB-N10 Nano]".