intel-iommu: Fix lookup in add device

We can't assume this device exists, fall back to the bridge itself.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Tested-by: Matthew Thode <prometheanfire@gentoo.org>
Cc: stable@vger.kernel.org
Signed-off-by: Joerg Roedel <joro@8bytes.org>