ath9k: use REG_RMW and rmw buffer in ath9k_hw_4k_set_gain

it is possible to reduce time needed for this function
by rplacing REG_WRITE with REG_RMW (plus dummy 0) and putt all commands
in same buffer.

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