ath: Speedup key set/reset ops for HTC driver

By enabling buffered register write for ath9k_htc driver
avoids unnecessary dissociation while rekeying phase under
heavy traffic exchange.

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