rt2x00: mt7620: differentiate based on SoC's CHIP_VER

rt2x00: mt7620: differentiate based on SoC's CHIP_VER

The vendor driver does things differently based on what it finds in the
SoC's CHIP_VER register, which should tell whether this is MT7620N or
MT7620A (PKG) and probably also the revision (VER) and most likely
also something about the silicon implementer (ECO).
Introduce codepaths just like the ones in the vendor driver to handle
the different chips properly.

Some of those paths are most likely dead code and left-overs from FPGA
versions or early prototypes of the chip. It'd thus be great if people
can post their kernel logs, at least the line telling the chip version
and eco, so we know what's actually out there in the wild -- all I
could find is
[ 0.000000] SoC Type: Ralink MT7620A ver:2 eco:6
and
[ 0.000000] SoC Type: Ralink MT7620N ver:2 eco:6
which would make things easier, as then we really just need to know
whether it's MT7620N or MT7620A and not care about FPGA or prototypes
with ver <= 1 and eco <= 2.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>