iwlwifi: change WARN_ON to IWL_WARN in iwl_mac_add_interface

We can start restarting firmware or RF kill switch can be turned on
during call to iwl_mac_add_interface(). That are normal working
conditions, so do not print call trace, just print simple message
instead.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/iwlwifi/iwl-core.c

@@ -2003,7 +2003,8 @@ int iwl_mac_add_interface(struct ieee80211_hw *hw, struct ieee80211_vif *vif)
 
 	mutex_lock(&priv->mutex);
 
-	if (WARN_ON(!iwl_is_ready_rf(priv))) {
+	if (!iwl_is_ready_rf(priv)) {
+		IWL_WARN(priv, "Try to add interface when device not ready\n");
 		err = -EINVAL;
 		goto out;
 	}