mt76: connac: set 6G phymode in single-sku support

Configure tx rate power for 6GHz channels. This is a preliminary patch
to enable 6GHz band for mt7921 devices.

Tested-by: Deren Wu <deren.wu@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>