ath9k: Fix incorrect sequence numbering for unaggregated QoS Frame.

This patch fixes an issue with the sequence numbers of unaggregated
QoS frames, because of which the frames are handled in a different order
at the AP and resulted in MLME REPLAYFAILURE.

Signed-off-by: Senthil Balasubramanian <senthilkumar@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>