Do not update slot number for faulty devices

For faulty or timeout devices MD will set the slot number
to -1, leaving us with no idea where the device was originally
located at.
So we should avoid updating the slot number in these cases;
we'll be updating to the correct slot number once rebuilding
has completed.

References: bnc#806508

Signed-off-by: Hannes Reinecke <hare@suse.de>