mac80211: dont assign seqno to or aggregate QoS Null frames

802.11 says:
"Sequence numbers for QoS (+)Null frames may be
set to any value."

However, if we use the normal counters then peers
will get confused with aggregation since there'll
be holes in the sequence number sequence.

To avoid that, neither assign a sequence number
to QoS null frames nor put them on aggregation.

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