ath9k: ath9k_hw_analog_shift_rmw: use REG_RMW

use REG_RMW in ath9k_hw_analog_shift_rmw.
It will double execution speed on usb bus.

Signed-off-by: Oleksij Rempel <linux@rempel-privat.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>