s390x/ipl: we always have an ipl device

Both s390 machines unconditionally create an ipl device, so no need to
handle the missing case.

Now we can also change s390_ipl_update_diag308() to return void.

Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>