s390/mm: fix MAX_DMA_ADDRESS physical vs virtual confusion

MAX_DMA_ADDRESS is defined and treated as a physical address,
whereas it should be virtual.

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