ath9k: Fix bug in TX aggregation

mac80211 expects the driver to fill in the starting
sequence number of an ADDBA request to initiate TX aggregation.
IEEE80211_TX_CTL_AMPDU would be set for frames only after a
successful ADDBA exchange, but we have to increment the
internal sequence counter for the normal(non-AMPDU) data frames proerly.

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