If there is no GSupplicantInterface set for any of the WiFi devices do
not report tethering enabling as EOPNOTSUPP which causes a disabling
notification for tethering. It will tear down the firewall set for the
tethering. Instead, do nothing and return 0. Tethering will be
intitiated by src/tethering.c: __connman_tethering_set_enabled().

Also fix iptables test. -m udp -m multiport passes checks now but is
wrong and iptables calls exit().