mac80211: assign seqnums for group QoS frames

According to 802.11-2012 9.3.2.10, paragraph 4, QoS
data frames with a group address in the Address 1 field
have sequence numbers allocated from the same counter
as non-QoS data and management frames.  Without this
flag, some drivers may not assign sequence numbers, and
in rare cases frames might get dropped.  Set the control
flag accordingly.

Signed-off-by: Bob Copeland <bob@cozybit.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>