mt76: mt76u: move mcu buffer allocation in mt76x02u drivers

Move mcu buffer allocation in mt76x2u/mt76x0u drivers since newer
chipsets (e.g. mt7663u) does not rely on synchronous mcu communication

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>