mt76: mt7615: introduce uni cmd command types

Introduce mcu uni command type. Uni commands rely on a stripped verions
of mt7615_mcu_txd data strutture. Split mt7615_mcu_txd_common and
mt7615_mcu_txd. Uni commands will be use by mt7663e driver

Co-developed-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>