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.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>