s390/pci: return correct dma address for offset > PAGE_SIZE

For offset > PAGE_SIZE, s390_dma_map_pages() will issue a warning
and return a wrong dma address.

This patch removes the warning and fixes the dma return address
calculation.

Signed-off-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>