ath9k: Use sta_state() callback

Instead of using the sta_add()/sta_remove() callbacks,
use the sta_state() callback since this gives
more fine-grained control.

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