ar71xx: ens202ext: Fix VLAN switch

ar71xx: ens202ext: Fix VLAN switch

The target ENS202EXT was just recently added right before the stable
release of Openwrt 18.

It flashes fine, but the physical switch is almost impossible to use
until you have a VLAN set up. Tested on two devices.

The actual problem is that eth0 represents nothing for whatever reason.
In other words, both WAN and LAN are running from eth1. There may be an
underlying problem in the build, but for now, I assume that this is
correct and that a VLAN switch is an appropriate fix.

Also, it's virtually impossible to get the switch running right through
LuCI. It is one thing to get a switch to appear, but attempting to
configure it breaks the whole thing. The VLAN has to be set up
perfectly, otherwise, interfaces will not start up, and one is forced to
reset settings, OR, the new LuCI feature kicks in and reverses any
steps. It is extremely difficult to determine which virtual ports
correspond to which physical ethernet ports without being able to set up
the switch in LuCI.

Temporary Workaround: followed directions here
[openwrt/luci#867](https://github.com/openwrt/luci/issues/867)

Reviewed-by: Marty Plummer <hanetzer@startmail.com>
Signed-off-by: Michael Pratt <mpratt51@gmail.com>
[commit author fix, subject fix, message text wrap]
Signed-off-by: Petr Štetiar <ynezz@true.cz>