ath9k: use REG_RMW and rmw buffer in ath9k_hw_4k_set_board_values

replace REG_WRITE to REG_RMW and place every thing in one
RMW buffer.

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