s390/zcrypt: cleanup AP bus timer code

Set the configuration timer at the end of the ap_scan_bus function.
Make use of setup_timer and remove some unnecessary add_timer, mod_timer
and del_timer_sync calls. Replace the complicated timer_pending, mod_timer
and add_timer code in ap_config_time_store with a simple mod_timer.

Reviewd-by: Ingo Tuchscherer <ingo.tuchscherer@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>