[SCSI] Fix Ultrastor asm snippet

Commit 1292500b replaced

"=m" (*field) : "1" (*field)

with

"=m" (*field) :

with comment "The following patch fixes it by using the '+' operator on
the (*field) operand, marking it as read-write to gcc."
'+' was actually forgotten.  This really puts it.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Cc: stable@kernel.org
Signed-off-by: James Bottomley <jbottomley@parallels.com>