mt76: connac: add 6GHz support to mt76_connac_mcu_uni_add_bss

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>