s390/smp: clean up a condition

I can never remember precedence rules.  Let's add some parenthesis so
this code is more clear.
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/kernel/smp.c

@@ -887,7 +887,7 @@ void __init smp_fill_possible_mask(void)
 
 	sclp_max = max(sclp.mtid, sclp.mtid_cp) + 1;
 	sclp_max = min(smp_max_threads, sclp_max);
-	sclp_max = sclp.max_cores * sclp_max ?: nr_cpu_ids;
+	sclp_max = (sclp.max_cores * sclp_max) ?: nr_cpu_ids;
 	possible = setup_possible_cpus ?: nr_cpu_ids;
 	possible = min(possible, sclp_max);
 	for (cpu = 0; cpu < possible && cpu < nr_cpu_ids; cpu++)