block, sx8: fix pointer math issue getting fw version

"mem" is type u8.  We need parenthesis here or it screws up the pointer
math probably leading to an oops.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: stable@kernel.org
Acked-by: Jeff Garzik <jgarzik@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>