mt76: introduce tx_queue_skb function pointer in mt76_bus_ops

Add tx_queue_skb function pointer in mt76_bus_ops since mt76x2u based
devices do not map mt76x2_txwi on dma buffers and it is not possible
to reuse mt76_dma_tx_queue_skb() routine to enqueue tx frames to
hw buffers

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>