ath9k: use swap() to make code cleaner

Using swap() make it more readable.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Yang Guang <yang.guang5@zte.com.cn>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20211028010451.7754-1-yang.guang5@zte.com.cn