mt76: usb: use mt76x02u_tx_prepare_skb to fill txwi

Use mt76x02u_tx_prepare_skb routine to fill txwi in mt76x2u and
mt76x0u driver and remove duplicated code. Moreover add static
qualifier to mt76x02_mac_tx_rate_val and mt76x02_mac_fill_txwi
routines

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