wl1271: Multicast filtering configuration

Enable multicast filtering. This way by default no multicast frames will
reach the host, and when needed, only required multicast frames can be
passed from the WLAN chipset to the host.

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Reviewed-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>