staging:r8188eu: inline unprotect_frame() in mon_recv_decrypted_recv()

It is useful to remove IV/ICV from rtl88eu_mon_recv_hook().
Also unprotect_frame() will be very short without skb_(pull|trim).

Signed-off-by: Ivan Safonov <insafonov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>