[SCSI] scsi_debug: fix invalid value check for guard module parameter

In the module initialization, invalid value for guard module parameter
is detected by the following check:

        if (scsi_debug_guard > 1) {
                printk(KERN_ERR "scsi_debug_init: guard must be 0 or 1\n");
                return -EINVAL;
        }

But this check isn't enough, because the type of scsi_debug_guard is
'int' and scsi_debug_guard could be a negative value.

This fixes it by changing the type of scsi_debug_guard to 'unsigned int'
instead of adding extra check for a negative value.

Reported-by: Joe Perches <joe@perches.com>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Acked-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>