ramips/mt7620: fix portmap based on board.d port assignment

ramips/mt7620: fix portmap based on board.d port assignment

When comparing to the port assignment in board.d/02_network, many
devices seem to use the wrong setup of mediatek,portmap.

The corrects the values for mt7620 subtarget based on the location
of the wan port.

A previous cleanup of obviously wrong values has already been done in
d3c0a944059b ("ramips: mt7620/mt7621: remove invalid mediatek,portmap")

Cc: Sungbo Eo <mans0n@gorani.run>
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>