mac80211: don't attempt to reorder multicast frames

Multicast frames can't be transmitted as part of an aggregation
session (such a session couldn't even be set up) so don't try to
reorder them. Trying to do so would cause the reorder to stop
working correctly since multicast QoS frames (as transmitted by
the Aruba APs this was found with) would cause sequence number
confusion in the buffer.

Cc: stable@vger.kernel.org
Reported-by: Blaise Gassend <blaise@suitabletech.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>