mt76x02: use commmon add interface for mt76x2u

Since we now support mt76x2u feature to allow set mac address
when creating interface in common code we can use it for
mt76x2u.

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