iwlagn: don't assign seqno to 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, don't assign sequence numbers to
QoS Null frames.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi GUy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>