PCI quirk: AMD 780: work around wrong vendor ID on APC bridge

In all AMD 780 family northbridges, the vendor ID of the internal
graphics PCI/PCI bridge reads not as AMD but as that of the mainboard
vendor, because the hardware actually returns the value of the subsystem
vendor ID (erratum 18).

We currently have additional quirk entries for Asus and Acer, but it is
likely that we will encounter more systems with other vendor IDs.

Since we do not know in advance all possible vendor IDs, a better way to
find the device is to declare the quirk on the host bridge, whose ID is
always correct, and use that device as a stepping stone to find the PCI/
PCI bridge, if present.

Reviewed-by: Matthew Wilcox <willy@linux.intel.com>
Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>