[PATCH 1/3] wifi: mt76: connac: use a helper to cache txpower_cur

[Lucid Duck]

Sean,

Tested the full 3-patch series on MT7921AU. All three bands working
correctly -- txpower reporting is fixed.

Test environment:

  Kernel:     6.19.8-200.fc43.x86_64 (Fedora 43)
  Device:     MediaTek MT7921AU USB (0e8d:7961)
  Firmware:   HW/SW 0x8a108a10, WM 20251223091148
  AP:         Tri-band (2.4/5/6 GHz)
  Regdomain:  CA (DFS-FCC), also tested US, JP, DE
  Note:       Internet maintained via separate USB ethernet
              adapter during WiFi testing

Baseline (stock 6.19.8, pre-patch):

  txpower 3.00 dBm on all bands

Post-patch txpower:

  2.4 GHz  ch 1   (2412 MHz, 20 MHz):   36.00 dBm
  5 GHz    ch 100 (5500 MHz, 80 MHz):    26.00 dBm
  6 GHz    ch 5   (5975 MHz, 80 MHz):    12.00 dBm

All values match expected CA regulatory limits. Connectivity verified
with 0% packet loss on every band.

Stress testing (28 tests, 28 passed, 0 failed):

  - Connect/disconnect: 10 cycles per band (30 total), txpower
    correct and ping 0% loss on every cycle
  - Band switching: 30 sequential hops (2.4 -> 5 -> 6 -> repeat),
    txpower correct on every hop, ping 30/30
  - Module reload: 15 full rmmod/insmod cycles of all mt76 modules,
    txpower correct after every reload
  - Throughput: 100-ping per band, 0% loss on all
      2.4 GHz: avg 2.4 ms, max 23.6 ms
      5 GHz:   avg 1.9 ms, max 8.0 ms
      6 GHz:   avg 2.2 ms, max 3.3 ms
  - Soak: 20-minute continuous test per band (60 min total),
    txpower sampled every 30 seconds, never deviated
      2.4 GHz: 40/40 samples at 36.00 dBm, slab delta -4 kB
      5 GHz:   40/40 samples at 26.00 dBm, slab delta +16 kB
      6 GHz:   40/40 samples at 12.00 dBm, slab delta -16 kB
  - Memory: 50 connect/disconnect cycles, slab delta +64 kB
    (negligible, no leak)
  - Band-hop marathon: 200 random band hops, 200/200 pass,
    ping 200/200 OK, completed in 1232 seconds
  - Concurrent managed + monitor: txpower stable on all bands
    with a monitor VIF present, ping 0% loss
  - Regulatory domain switching: txpower correctly adjusts
    per country on 5 GHz
      CA: 26.00 dBm
      US: 24.00 dBm
      JP: 23.00 dBm
      DE: 26.00 dBm

Zero kernel warnings in dmesg throughout all testing.

For context, I originally reported this bug and submitted a fix in
January (v1 through v4 on linux-wireless, as Lucid Duck). Patches 1
and 2 in this series address the same core issue -- capturing the rate
power loop output and subtracting the path delta before caching
txpower_cur. Patch 3 (the per-vif-link get_txpower callback) is a
good addition that was not in my series.

Reported-by: Lucid Duck <lucid_duck at justthetip.ca>
Tested-by: Lucid Duck <lucid_duck at justthetip.ca>