[SCSI] ncr53c8xx: div reaches -1

With while(--div >= 0) { ... } div reaches -1.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>