s390/ap: add missing virt_to_phys address conversion

The address of the notification-indicator byte is an absolute
address. Therefore convert its virtual to a physical address before
being used with PQAP(AQIC).

Note: this currently doesn't fix a real bug, since virtual addresses
are indentical to physical ones.

Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Reviewed-by: Harald Freudenberger <freude@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>