mt76: mt76u: introduce mt76u_skb_dma_info routine

Introduce mt76u_skb_dma_info utility routine in mt76-usb module in order
to be reused adding mt7663u support

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>