iommu/intel: Fix ACS path checking

SR-IOV can create buses without a bridge.  There may be other cases
where this happens as well.  In these cases skip to the parent bus
and continue testing devices there.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Tested-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>