scsi: ignore errors from scsi_dh_add_device()

device handler initialisation might fail due to a number of
reasons. But as device_handlers are optional this shouldn't
cause us to disable the device entirely.
So just ignore errors from scsi_dh_add_device().

Reviewed-by: Johannes Thumshirn <jthumshirn@suse.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>