ath9k: Assign seq# when mac80211 requests this

Use TX control flag IEEE80211_TX_CTL_ASSIGN_SEQ as a request to update
the seq# for the frames. This will likely require some further cleanup
to get seq# correctly for Beacons vs. other frames and also potentially
for multiple BSSes. Anyway, this is better than ending up sending out
most frames with seq# 0.

Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>