mt76x0: introduce mt76x0_tx_prepare_skb routine

Add mt76x0_tx_prepare_skb routine as tx txwi handler.
mt76x0_tx_prepare_skb will be used by mt76-usb layer

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>