s390/sclp: Use monotonic clock in sclp_sync_wait()

sclp_sync_wait() should use the monotonic clock for the delay loop.
Otherwise the code won't work correctly when the clock is changed.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>