mt76: move mt76x2u_tx_complete_skb in mt76x02-lib moudule

Move mt76x2u_tx_complete_skb routine in mt76x02-lib module
and rename it in mt76x02_tx_complete_skb in order to be reused
in mt76x0 driver

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>