networks: Fetch only one wan interface and ignore the others
Related to RT #1372462
(re)Foris currently expects only one interface in WAN.
However when someone attaches multiple interfaces to wan bridge, which can be configured in LuCI, (re)Foris throws errors.
This aims to implement simple compatibility workaround so (re)Foris won't throw error and just work with only one wan interface (ignores the other ones).