realtek: revert to "standard" management configuration

The default management interface should be easy to find for users
doing "blind" installations without console access.  There are
already multiple examples in the forum of advanced early adopters
having problems locating the management interface after installing
OpenWrt.

Requiring tagged VLAN configration to access the initial management
interface creates unnecessary hassle at best. Errors on the other
end are close to impossible to debug without console access, even
for advanced users.  Less advanced users might have problems with
the concept of VLAN tagging.

Limiting management access to a single arbitrary port among up to
52 possible LAN ports makes this even more difficult, for no
reason at all. Users might have reasons to use a different port
for management.  And they might even have difficulties using the
OpenWrt selected one. The port might be the wrong type for their
management link (e.g copper instead of fibre).  Or they might
depend on PoE power from a device which they can't reconfigure.

User expectations will be based on
- OpenWrt defaults for other devices
- stock firmware default for the device in question
- common default behaviour of similar devices

All 3 cases point to a static IP address accessible on the native
VLAN of any LAN port.  A switch does not have any WAN port.  All
ports are LAN ports.

This changes the default network configuration in line with these
expectations.

Cc: John Crispin <john@phrozen.org>
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Acked-by: Hauke Mehrtens <hauke@hauke-m.de>