ath9k: drop redundant code in ar9003_hw_set_channel

AR9330, AR9485, AR9531, AR9550, AR9561 and AR9565 all use same
channel set register configuration which allows for small code
size reduction.

Signed-off-by: Tomislav Požega <pozega.tomislav@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>