mt76: mt76u: introduce mt76u_alloc_mcu_queue utility routine

Add mt76u_alloc_mcu_queue  utility routine to allocate mcu hw rx queue.
This is a preliminary patch to support new devices (e.g. mt7663u) that
rely on a hw queue for mcu messages

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>