[PR#4111] mt7620: fix ethernet with external switches

[Michael]

My PR needs someone to review, and possibly test if they want to.

https://github.com/openwrt/openwrt/pull/4111

For 2 MT7620 boards in master and 21.02 that have an external switch,
the situation for ethernet is critical, in other words, no ethernet functionality at all.
This is due to a VLAN issue that is resolved in the last commit.
However, even with the VLAN fix, only some of the ports function.
The rest of the commits in total fix that issue,
as well as other small corrections to the driver.

Also, there are other similar devices that potentially could be added if this gets resolved,
but currently have the same issue, (for example PR#4112).

I have tested the patches on a board with an external switch, and one without.
Several others have also tested the patches (see discussion in the PR).

Thanks,
Michael Pratt