mt76x0: move stop related routines in mt76x0_mac_stop

Move tear-down routines in mt76x0_mac_stop function.
mt76x0_mac_stop routines will be reused in mt76x0_suspend

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>