mt76: mt7615: add mt7615_mcu_ops data structure

Introduce mt7615_mcu_ops data structure in order to support multiple
mcu ops API.
Move mt7615_mcu_set_{tx,rx}_ba to mt7615_mcu_ops differentiating between
fw v1 and v2. This is a preliminary patch to properly support mt7663e
firmware.
Rework utility routines to rely on skb APIs for msg parsing

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>