intel-iommu: Fix device-to-iommu mapping for PCI-PCI bridges.

When the DMAR table identifies that a PCI-PCI bridge belongs to a given
IOMMU, that means that the bridge and all devices behind it should be
associated with the IOMMU. Not just the bridge itself.

This fixes the device_to_iommu() function accordingly.

(It's broken if you have the same PCI bus numbers in multiple domains,
but this function was always broken in that way; I'll be dealing with
that later).

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>