s390/pci_dma: simplify dma address calculation

Simplify the code we use to calculate dma addresses by putting
everything related in a dma_alloc_address function. Also provide
a dma_free_address counterpart.

Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Reviewed-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>