mac80211: fix multicast LED blinking and counter

As multicast-frames can't be fragmented, "dot11MulticastReceivedFrameCount"
stopped being incremented after the use-after-free fix. Furthermore, the
RX-LED will be triggered by every multicast frame (which wouldn't happen
before) which wouldn't allow the LED to rest at all.

Fixes https://bugzilla.kernel.org/show_bug.cgi?id=89431

 which also had the
patch.

Cc: stable@vger.kernel.org
Fixes: b8fff407 ("mac80211: fix use-after-free in defragmentation")
Signed-off-by: Andreas Müller <goo@stapelspeicher.org>
[rewrite commit message]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>