wifi: mt76: connac: use a helper to cache txpower_cur

wifi: mt76: connac: use a helper to cache txpower_cur

The cached txpower value is derived from the bounded channel power after
applying the chainmask path delta.

Use a helper for that conversion so callers do not open-code it.

--
v2:
- Rebased onto the latest mt76 tree
- Added Reported-by, Tested-by, Co-developed-by and Signed-off-by tags

Reported-by: Devin Wittmayer <lucid_duck@justthetip.ca>
Closes: https://lore.kernel.org/linux-wireless/20260130215839.53270-1-lucid_duck@justthetip.ca/
Tested-by: Devin Wittmayer <lucid_duck@justthetip.ca>
Tested-by: Satadru Pramanik <satadru@gmail.com>
Co-developed-by: Devin Wittmayer <lucid_duck@justthetip.ca>
Signed-off-by: Devin Wittmayer <lucid_duck@justthetip.ca>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Reported-by: Lucid Duck <lucid_duck@justthetip.ca>
Tested-by: Lucid Duck <lucid_duck@justthetip.ca>
Link: https://patch.msgid.link/20260401182322.64355-1-sean.wang@kernel.org
Signed-off-by: Felix Fietkau <nbd@nbd.name>