mt76: move mt76x2u_set_txinfo in mt76x02-lib module

Move mt76x2u_set_txinfo routine in mt76x02-lib module and rename it in
mt76x02_set_txinfo 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>