scsi_debug: use pdt constants

Use TYPE_* constants for SCSI peripheral device types instead of
numbers. Further cleanups requested by checkpatch.pl.
Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
Reviewed-by: Hannes Reinicke <hare@suse.de>
Reviewed-by: Bart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>