s390/pci/dma: use correct segment boundary size

The boundary size for iommu_area_alloc() is currently set to a constant
value. This is wrong, we shouldn't use a constant value but rather the
return value of dma_get_seg_boundary(), since a device driver can override
the default.

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