ath9k_htc: add support of channel switch

Add the support of channel switching functionality, similar
to ath9k support.

Tested with TP-Link TL-WN722N and TL-WN821N.

Signed-off-by: Chun-Yeow Yeoh <yeohchunyeow@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>