[SCSI] sd: DIF: Fix tagging on platforms with signed char

Switch tag arrays to u8 to prevent problems on platforms with signed
char.

Reported-by: Tim LaBerge <tim.laberge@Quantum.Com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>