mt76: connac: fix last_chan configuration in mt76_connac_mcu_rate_txpower_band

last_ch configuration must not be dependent on the current configured band
but it is defined by hw capabilities since the fw always expects the
following order:
- 2GHz
- 5GHz
- 6GHz

Fixes: 9b2ea8ee ("mt76: connac: set 6G phymode in single-sku support")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>