mt76: fix rounding issues on converting per-chain and combined txpower

Unify code converting between the different txpower values. Always add/remove
the combined txpower delta before dividing half-dB values.
Also fix the combined txpower delta values. The correct half-dB delta for
3 chains is 9, not 8.

Signed-off-by: Felix Fietkau <nbd@nbd.name>