gluon-core: disable multicast snooping on the WAN bridge

This seems to mitigate weird bridge forwarding issues.

package/gluon-core/files/lib/gluon/upgrade/110-network

@@ -9,6 +9,7 @@ uci:section('network', 'interface', 'wan',
 	    {
 	      ifname = sysconfig.wan_ifname,
 	      type = 'bridge',
+	      igmp_snooping = 0,
 	      peerdns = 0,
 	      auto = 1,
 	    }