ath9k: Fix bug in rate control capability registration

Dual stream capability must be registered only when the
hardware supports it.

Signed-off-by: Sujith <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/ath9k/rc.c

@@ -1358,7 +1358,9 @@ static void ath_rc_init(struct ath_softc *sc,
 	}
 
 	if (sta->ht_cap.ht_supported) {
-		ath_rc_priv->ht_cap = (WLAN_RC_HT_FLAG | WLAN_RC_DS_FLAG);
+		ath_rc_priv->ht_cap = WLAN_RC_HT_FLAG;
+		if (sc->sc_tx_chainmask != 1)
+			ath_rc_priv->ht_cap |= WLAN_RC_DS_FLAG;
 		if (sta->ht_cap.cap & IEEE80211_HT_CAP_SUP_WIDTH_20_40)
 			ath_rc_priv->ht_cap |= WLAN_RC_40_FLAG;
 		if (sta->ht_cap.cap & IEEE80211_HT_CAP_SGI_40)