mt76x2: convert between per-chain tx power and combined output

Using both chains adds max. 3 dBm. A similar worst-case calculation is
being used in ath9k as well to ensure that the hardware stays within
regulatory limits

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>