Host bridge drivers do not use the portdrv interfaces (struct pcie_device,
struct pcie_port_service_driver, pcie_port_service_register(), etc), and
they should not select CONFIG_PCIEPORTBUS.

If users need the portdrv services, they can select CONFIG_PCIEPORTBUS just
like all other PCI users.
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>