ath9k: dfs: use swap macro in ath9k_check_chirping

Make use of the swap macro and remove unnecessary variable temp.
This makes the code easier to read and maintain.

This code was detected with the help of Coccinelle.

Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>