mt76: move mt76_connac2_mcu_fill_message in mt76_connac module

Move mt76_connac2_mcu_fill_message routine in shared module in order to
reuse it for mt7921 and mt7915e drivers. This is a preliminary patch to
add mt7990 driver support.

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