mac80211: fix allmulti/promisc behaviour

When an interface with promisc/allmulti bit is taken down,
the mac80211 state can become confused. This fixes it by
making mac80211 keep track of all *active* interfaces that
have the promisc/allmulti bit set in the sdata, we sync
the interface bit into sdata at set_multicast_list() time
so this works.

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