device: fix changing firewall zone during _set_state_full()

For regular devices that don't have a separate ip_iface/ip_ifindex,
the ip_ifindex is left at zero. Hence, the condition is always
true and does not work as intended, resulting in setting the
firewall zone twice.

Fixes: 7cf5c326bc

src/devices/nm-device.c

@@ -12663,7 +12663,7 @@ _set_state_full (NMDevice *self,
 		applied_connection = nm_device_get_applied_connection (self);
 
 		if (   applied_connection
-		    && priv->ifindex != priv->ip_ifindex
+		    && priv->ip_iface
 		    && !nm_device_sys_iface_state_is_external (self)) {
 			NMSettingConnection *s_con;
 			const char *zone;