iwl4965: fix return code indicating one interface is supported

This is a fix to patch "iwlwifi: fix iwl_mac_add_interface handler".
In that patch the return code was corrected for iwl3945, but not for
iwl4965.
Signed-off-by: NReinette Chatre <reinette.chatre@intel.com>
Cc: Tomas Carnecky <tom@dbservice.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/wireless/iwlwifi/iwl4965-base.c

@@ -7448,7 +7448,7 @@ static int iwl4965_mac_add_interface(struct ieee80211_hw *hw,
 
 	if (priv->vif) {
 		IWL_DEBUG_MAC80211("leave - vif != NULL\n");
-		return 0;
+		return -EOPNOTSUPP;
 	}
 
 	spin_lock_irqsave(&priv->lock, flags);