mt76: mmio: move mt76_insert_hdr_pad in mt76_dma_tx_queue_skb

Introduce tx_aligned4_skbs in mt76_driver_ops and move
mt76_insert_hdr_pad in mt76_dma_tx_queue_skb. This is a preliminary
patch in order to unify tx dma mapping for mt76x02 and new chipsets

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