iommu/vt-d: Don't return error when device gets right domain

If a device gets a right domain in add_device ops, it shouldn't
return error.

Fixes: 942067f1 ("iommu/vt-d: Identify default domains replaced with private")
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>