smartd service: never quit to avoid service failure if no dev

smartd service: never quit to avoid service failure if no dev

If no devices to monitor are found smartd exits, this avoids that by
telling it to never exit, while it's not really really ideal we do it
this way for now to ensure that upgrades can work.
Config errors get also somewhat ignored with this :/

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>