[S390] zcrypt: fix scheduling of hrtimer ap_poll_timer

Every time a request is enqueued or there is some work outstanding
from the ap_tasklet, the ap_poll_timer is scheduled again.
Unfortunately it was permanently called. It looked as if it was
started in the past and thus imediately expired.
This has been changed. First it is checked if the hrtimer is already
expired. Then the expiring time is forwarded and the timer restarted.

Signed-off-by: Felix Beck <felix.beck@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>