ath9k: use WARN_ON_ONCE in ath_rc_get_highest_rix

The device seems to survive the issue, so no need to flood the logs
about it...
Signed-off-by: John W. Linville <linville@tuxdriver.com>

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

@@ -694,7 +694,7 @@ static u8 ath_rc_get_highest_rix(struct ath_softc *sc,
 		return rate;
 
 	/* This should not happen */
-	WARN_ON(1);
+	WARN_ON_ONCE(1);
 
 	rate = ath_rc_priv->valid_rate_index[0];