iommu/s390: Constify iommu_ops

iommu_ops are not supposed to change at runtime.
Functions 'bus_set_iommu' working with const iommu_ops provided
by <linux/iommu.h>. So mark the non-const structs as const.

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Acked-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>