ath9k: replace ternary operator with max()

Fix the following coccicheck warning:

drivers/net/wireless/ath/ath9k/dfs.c:249:28-30: WARNING
opportunity for max()

Signed-off-by: Guo Zhengkui <guozhengkui@vivo.com>
Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20220517024106.77050-1-guozhengkui@vivo.com