octeon: use dedicated function to move config backup

octeon: use dedicated function to move config backup

All octeon devices use the same or a very similar way to backup
and restore configuration.

We expect to have more devices added and in order to stop
repeating ourselves move the logic to a separate function.

While at it, add a few checks.

Signed-off-by: Roman Kuzmitskii <damex.pp@icloud.com>
[commit message facelift]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>