ramips: erx and erx-sfp: fix missing WAN interface

ramips: erx and erx-sfp: fix missing WAN interface

This partially reverts commit 5acd1ed0be0d ("ramips: mt7621: fix
Ubiquiti ER-X ports names and MAC addresses"), this change was discussed
in https://github.com/openwrt/openwrt/pull/2901#discussion_r407238452

With commit 5acd1ed0be0d ("ramips: mt7621: fix Ubiquiti ER-X ports names
and MAC addresses"), all the ports were put into the LAN bridge, with
the argument that the OEM firmware does not have a WAN port enabled.  In
the default OEM setup, all of the ports except eth0 are dead and eth0 is
set to a static IP address without providing DHCP services when
connected.  It is only after the wizard has been run that eth0 becomes
the WAN port and all the rest of the ports belong to LAN with DHCP
enabled.

Having all of the ports set to the LAN bridge does not mirror the default
OEM setup.  To accomplish that, then only eth0 would be in the LAN bridge.
But this is not the expected behaviour of OpenWrt.

Therefore this proposal to set eth0 to WAN and eth1-N to LAN provides
the expected behaviour expected from OpenWrt, maintains the current
documentation as up-to-date, and does not require the user to manually
detach eth0 from the LAN bridge, create the WAN(6) interface(s), and set
eth0 to the WAN(6) interface(s).

Fixes: 5acd1ed0be0d ("ramips: mt7621: fix Ubiquiti ER-X ports names and MAC addresses")
Signed-off-by: Perry Melange <isprotejesvalkata@gmail.com>
[commit subject and description tweaks]
Signed-off-by: Petr Štetiar <ynezz@true.cz>