PCI: populate subsystem vendor and device IDs for PCI bridges

Change to populate the subsystem vendor and subsytem device IDs for
PCI-PCI bridges that implement the PCI Subsystem Vendor ID capability.
Previously bridges left subsystem vendor IDs unpopulated.

Signed-off-by: Gabe Black <gabe.black@ni.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>