s390: cleanup timer API use

cleanup the s390's use of the timer API
- del_timer() contains timer_pending() condition
- mod_timer(timer, expires) is equivalent to:

	del_timer(timer);
	timer->expires = expires;
	add_timer(timer);

If the timer is inactive it will be activated, using add_timer() on
condition !timer_pending(&private->timer) is redundant.

Just cleanup, no logic change.

Signed-off-by: Yu Liao <liaoyu15@huawei.com>
Link: https://lore.kernel.org/r/20220322030057.1243196-1-liaoyu15@huawei.com


Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>