scsi: sd: Remove a useless comparison

This patch avoids that gcc reports the following warning when
building with W=1:

drivers/scsi/sd.c:315:10: warning: comparison of unsigned expression >= 0 is always true [-Wtype-limits]
  if (val >= 0 && val <= T10_PI_TYPE3_PROTECTION)

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