mac80211: don't inspect Sequence Control field on control frames

Per IEEE Std. 802.11-2012, Sec 8.2.4.4.1, the sequence Control field is
not present in control frames.  We noticed this problem when processing
Block Ack Requests.

Cc: stable@vger.kernel.org
Signed-off-by: Javier Cardona <javier@cozybit.com>
Signed-off-by: Javier Lopez <jlopex@cozybit.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>