ath5k: replace ternary operator with min()

Fix the following coccicheck warning:

drivers/net/wireless/ath/ath5k/phy.c:3139:62-63: WARNING
opportunity for min()

Signed-off-by: Guo Zhengkui <guozhengkui@vivo.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20220517023923.76989-1-guozhengkui@vivo.com