ramips: mt7620/mt7621: remove invalid mediatek,portmap
mt7620 and mt7621 use mt7530 driver, which only accepts "llllw", "wllll",
and "lwlll" values.
According to its switch setup, Mi Router 3G v2 has a WAN port at port 4,
so the correct value should be "llllw".
Signed-off-by: Sungbo Eo <[email protected]>