[PATCH v4] wifi: mt76: mt7921: fix txpower reporting from rate power configuration

[Lucid Duck]

Note for stable backport: this patch uses mt76_tx_power_path_delta()
which was introduced in commit 56e38675c5bd ("mt76: support power delta
calculation for 5 TX paths"). That commit renamed the older
mt76_tx_power_nss_delta() and is needed as a prerequisite for the
backport to compile.

For stable kernels that don't have 56e3867, a trivial substitution of
mt76_tx_power_nss_delta for mt76_tx_power_path_delta produces identical
results for 1-4 chain devices (the lookup tables match exactly).

Reported-by: sam8641 (compile failure on 6.12.74)