mac80211: fix ieee80211_set_multicast_list

mac80211: fix ieee80211_set_multicast_list

I recently experienced unexplainable behaviour with the b43
driver when I had broken firmware uploaded. The cause may have
been that promisc mode was not correctly enabled or disabled
and this bug may have been the cause.

Note how the values are compared later in the function so
just doing the & will result in the wrong thing being
compared and the test being false almost always.

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