ath9k: Fix TX fragmentation

Assigning sequence number for frames without taking care
of the fragment field breaks transmission of fragmented frames.
Fix this by assigning the fragment number properly.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>