rt2x00: Use addr_mask to disallow invalid MAC addresses in mutli-bssid mode

Reported-by: Petr Stetiar <ynezz@true.cz>
Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>