scsi: scsi_lib: rework scsi_internal_device_unblock_nowait()

Rework scsi_internal_device_unblock_nowait() into using a switch
statement. No functional changes.

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