mt76: mt7921: move mt7921_usb_sdio_tx_complete_skb in common mac code.

This is a preliminary patch to add mt7921u driver support.

Tested-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>