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

Author: Guo Zhengkui

drivers/net/wireless/ath/ath9k/dfs.c

@@ -246,7 +246,7 @@ ath9k_postprocess_radar_event(struct ath_softc *sc,
 		DFS_STAT_INC(sc, dc_phy_errors);
 
 		/* when both are present use stronger one */
-		rssi = (ard->rssi < ard->ext_rssi) ? ard->ext_rssi : ard->rssi;
+		rssi = max(ard->rssi, ard->ext_rssi);
 		break;
 	default:
 		/*