drivers/scsi/ncr53c8xx.c: fix warning

drivers/scsi/ncr53c8xx.c: In function 'process_waiting_list':
drivers/scsi/ncr53c8xx.c:8225: warning: suggest parentheses around assignment used as truth value

recently added by

commit 172c122df5186e7cbd413d61757ff90267331002
Author: Harvey Harrison <harvey.harrison@gmail.com>
Date:   Mon Apr 28 16:50:03 2008 -0700

    scsi: fix integer as NULL pointer warnings

Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
Cc: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>