device: fix reapply of MTU
When we set the MTU on the link we remember its previous source (ip-config, parent-device or connection profile) and don't change it again afterwards to avoid interfering with user's manual changes. The only exceptions when we change it again are (1) if the parent device MTU changes and (2) if the new MTU has higher priority than the one previously set.
To allow a live reapply of the MTU property we also need to clear the saved source, or the checks described above will prevent setting the new value.
Fixes: 2f891723 ('device: rework mtu priority handling')