wl1271: Fix multicast list handling

Fixes bugs in the multicast list handling:
 - Use atomic memory allocation in non-sleepable context
 - Increment address pointer when iterating list of MC addresses

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>