mac80211: allow sending multicast frames through virtual ports

When reworking the port access control code, I forgot multicast frames
and those are now always rejected because the destination station is
not known. This changes the code to allow through multicast frames and
also avoid the sta hash lookup (which is bound to fail) for them.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>