iommu/vt-d: Unlink device if failed to add to group

If the device fails to be added to the group, make sure to unlink the
reference before returning.

Signed-off-by: Jon Derrick <jonathan.derrick@intel.com>
Fixes: 39ab9555 ("iommu: Add sysfs bindings for struct iommu_device")
Acked-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>