Alfa AWUS036ACHM (MT7610U) incorrect txpower (set, displayed, and what's settable)

There are a number of issues with this adapter. Some related discussion can be found at

• https://github.com/openwrt/mt76/issues/633
• https://github.com/morrownr/USB-WiFi/issues/34

Please understand that while I have development experience, I'm not super familiar with hacking on OpenWRT yet.

Issues visible on SNAPSHOT as of this writing:

1. txpower may be set too low, see also above discussion
2. txpower may also be reported too low as reported above
3. the maximum txpower one can set in Luci is 14dBm
4. the dongle is detected as a "Sabrent NTWLAC 802.11ac/b/g/n"

I believe some of the issues come from the fact that the Alfa AWUS036ACHM has a separate RF amplifier, so max settable txpower and displayed txpower are not being properly scaled somehow.

The very low maximum txpower which may be observed in e.g. 2.4GHz band is likely related to the above-mentioned issues. I have tried hex editing mt76x0-common.ko as described at this comment but it did not improve issues for me.

Begin with MT7610U support and the mt76x0-common.ko path mentioned in the report, then review the linked mt76#633 discussion and reproduce the behavior on an OpenWrt SNAPSHOT with the Alfa adapter. Done means txpower is correctly set and reported, the valid maximum is exposed in LuCI, and the adapter is identified correctly.