ath9k: fix max phy rate at rate control init

The stations always chooses 1Mbps for all trasmitting frames,
whenever the AP is configured to lock the supported rates.
As the max phy rate is always set with the 4th from highest phy rate,
this assumption might be wrong if we have less than that. Fix that.

Cc: stable@kernel.org
Cc: Paul Stewart <pstew@google.com>
Reported-by: NAjay Gummalla <agummalla@google.com>
Signed-off-by: NRajkumar Manoharan <rmanohar@qca.qualcomm.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

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

@@ -1270,7 +1270,9 @@ static void ath_rc_init(struct ath_softc *sc,
 
 	ath_rc_priv->max_valid_rate = k;
 	ath_rc_sort_validrates(rate_table, ath_rc_priv);
-	ath_rc_priv->rate_max_phy = ath_rc_priv->valid_rate_index[k-4];
+	ath_rc_priv->rate_max_phy = (k > 4) ?
+					ath_rc_priv->valid_rate_index[k-4] :
+					ath_rc_priv->valid_rate_index[k-1];
 	ath_rc_priv->rate_table = rate_table;
 
 	ath_dbg(common, ATH_DBG_CONFIG,