ath9k_htc: Fix bug in aggregation initiation

Accessing the sta pointer in TX completion without
approprate RCU protection is wrong. Fix this.

Also, RCU protection is needed when the station's
aggregation state is updated. Handle this properly.

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