s390/vmcp: use page_to_virt instead of page_to_phys

Fix wrong usage of page_to_phys/phys_to_page.

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

Acked-by: Thomas Richter <tmricht@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>