PCI: Enable upstream bridges even for VFs on virtual buses

Previously we enabled the upstream PCI-to-PCI bridge only when
"dev->bus->self != NULL".  In the case of a VF on a virtual bus, where
"bus->self == NULL", we didn't enable the upstream bridge.

This fixes that by enabling the upstream bridge of the PF corresponding to
the VF.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Yinghai Lu <yinghai@kernel.org>