mt76: rename mt76_tx_queue_skb in mt76_dma_tx_queue_skb

Move mt76_dma_tx_queue_skb routine in dma.c.
Remove static qualifier from mt76_get_txwi definition

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