iommu: Revive device_group iommu-ops call-back

That call-back is currently unused, change it into a
call-back function for finding the right IOMMU group for a
device.
This is a first step to remove the hard-coded PCI dependency
in the iommu-group code.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
index 049df49..5637463 100644
--- a/drivers/iommu/iommu.c
+++ b/drivers/iommu/iommu.c
@@ -814,6 +814,7 @@
  */
 struct iommu_group *iommu_group_get_for_dev(struct device *dev)
 {
+	const struct iommu_ops *ops = dev->bus->iommu_ops;
 	struct iommu_group *group;
 	int ret;
 
@@ -821,10 +822,12 @@
 	if (group)
 		return group;
 
-	if (!dev_is_pci(dev))
-		return ERR_PTR(-EINVAL);
+	group = ERR_PTR(-EINVAL);
 
-	group = iommu_group_get_for_pci_dev(to_pci_dev(dev));
+	if (ops && ops->device_group)
+		group = ops->device_group(dev);
+	else if (dev_is_pci(dev))
+		group = iommu_group_get_for_pci_dev(to_pci_dev(dev));
 
 	if (IS_ERR(group))
 		return group;