scsi_debug: check for bigger value first

Even for signed types we have to check for bigger positive value first.
Otherwise it will be never happened.

Signed-off-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Ewan Milne <emilne@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>