Fix the subvendor ID for PCI-PCI bridges.

Fix the subvendor ID for PCI-PCI bridges.
- Retire the PCI_SUB*_1 constants and don't try to read a subvendor ID out
  of them.  There isn't a standard subvendor ID field for PCI-PCI bridges.
  Instead, the dword at offset 0x34 is actually mostly reserved except for
  the LSB which is the capabilities pointer.
- Add support for the PCI-PCI bridge subvendor ID capability (13) and use
  it to set the subvendor ID for PCI-PCI bridges.

MFC after:	 1 month