mt76 TX power not working in multiple chipsets (mt7610, mt7613, mt7615, mt7603)

Just looking at the bug reports here and in the OpenWRT defects repository, it is clear that for various mt76 chipsets the TX power is simply not working.

I myself am experimenting two issues with OpenWRT and mt76 drivers:

• With mt7613 on an TP-Link Archer C6 v3 with OpenWRT where TX power is set to maximum and cannot be changed (details in the OpenWRT bug FS#181).
• With mt7610 on an TP-Link RE200 v2 with OpenWRT TX power is too low (max 9dBm), cannot be changed above this and makes the 5Ghz radio unusable.

Searching here in the mt76 repository about similar TX power issues, I've found multiple similar reports:

TP-LINK RE200 problem with the operation of the WLAN chipset. OpenWRT rel 21.02rc4 #606
MT7610E: TX-Power Values incorrect -> low range #604
MT76x0: Correct VHT MCS 8/9 tx power eeprom offset #577
MT7615dn: 5G max power is 6dBm #574
Xiaomi r3p mt7615N low tx power 2ghz 5ghz #539
Low transmission power on MT7610EN #530
mt7603e max tx power is only 8 dBm on HiWiFi HC5962 #525
low tx power with mt76x0e driver #484
Low-incorrect txpower on Mediatek 7628 (TP-Link 840 ver 6.0) #279
Various issues with 5Ghz MT7610E #227
low tx power m7610e #216

And some other more.

While as in the TP-Link Archer C6 v3 (mt7613) where TX Power is stuck at maximum allows it to be perfectly usable, on other devices such as TP-Link RE200 v2 (mt7610) wifi is unusable due to the very weak 5Ghz signal power.

So instead of opening more bug reports here for my TX Power issues with mt7613 and mt7610, I've decided summarize the existing bug reports about TX power here so perhaps we can concentrate the developers' focus on this single bug report.

Start by reviewing the linked mt76 and OpenWrt reports for the affected mt7610, mt7613, mt7615, and mt7603 chipsets; the issue names no source files or tests. A complete outcome would identify the shared TX-power cause, define the needed driver changes, and verify configured power across the reported chipsets.