ath9k_htc: add new WMI_REG_RMW_CMDID command

Since usb bus add extra delay on each request, a command
with read + write requests is too expensive. We can dramtically
reduce usb load by moving this command to firmware.

In my tests, this patch will reduce channel scan time
for about 5-10 seconds.

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