time, s390: Get rid of compile warning

"s390: Use direct ktime path for s390 clockevent device" in linux-next
introduces this compile warning:

arch/s390/kernel/time.c: In function 's390_next_ktime':
arch/s390/kernel/time.c:118:2: warning:
  comparison of distinct pointer types lacks a cast [enabled by default]

Just use a u64 instead of an s64 variable. This is not a problem since it
will always contain a positive value.
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Link: http://lkml.kernel.org/r/1316675957-5538-1-git-send-email-heiko.carstens@de.ibm.comSigned-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/s390/kernel/time.c

@@ -112,7 +112,7 @@ static void fixup_clock_comparator(unsigned long long delta)
 static int s390_next_ktime(ktime_t expires,
 			   struct clock_event_device *evt)
 {
-	s64 nsecs;
+	u64 nsecs;
 
 	nsecs = ktime_to_ns(ktime_sub(expires, ktime_get_monotonic_offset()));
 	do_div(nsecs, 125);