rt2x00: never disable multicast because it disables broadcast too

On rt73 and rt61 disabling reception of multicast packets also disables
broadcast traffic which we never want to do. Therefore we should never
disable multicast.

Signed-off-by: Adam Baker <linux@baker-net.org.uk>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>