s390/zcrypt: use system work queue for ap_scan_bus

There is a system work queue system_long_wq for long running work.
Use this work queue for the AP bus scan loop.

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