iommu/vt-d: Fix get_domain_for_dev() handling of upstream PCIe bridges

Commit 146922ec ("iommu/vt-d: Make get_domain_for_dev() take struct
device") introduced new variables bridge_bus and bridge_devfn to
identify the upstream PCIe to PCI bridge responsible for the given
target device. Leaving the original bus/devfn variables to identify
the target device itself, now that it is no longer assumed to be PCI
and we can no longer trivially find that information.

However, the patch failed to correctly use the new variables in all
cases; instead using the as-yet-uninitialised 'bus' and 'devfn'
variables.

Reported-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>