When a new function is attached to an iommu domain we need to register
I/O address translation parameters. Since commit
69eea95c ("s390/pci_dma: fix DMA table corruption with > 4 TB main memory")
start_dma and end_dma correctly describe the range of usable I/O addresses.

Simplify the code by using these values directly.
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>