s390/time: use CLOCKSOURCE_MASK

Make use of CLOCKSOURCE_MASK instead of open-coding it.
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

arch/s390/kernel/time.c

@@ -253,7 +253,7 @@ static struct clocksource clocksource_tod = {
 	.name		= "tod",
 	.rating		= 400,
 	.read		= read_tod_clock,
-	.mask		= -1ULL,
+	.mask		= CLOCKSOURCE_MASK(64),
 	.mult		= 1000,
 	.shift		= 12,
 	.flags		= CLOCK_SOURCE_IS_CONTINUOUS,