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

[Lucid Duck]

Adding Tested-by from Satadru Pramanik, who tested the full 3-patch
series on MT7925U with kernel 7.0-rc5 (US regdomain):

Before patch (5 GHz ch 36, 80 MHz):  txpower 3.00 dBm
After patch  (5 GHz ch 36, 80 MHz):  txpower 23.00 dBm

Tested-by: Satadru Pramanik <satadru at gmail.com>