PCI: Add pci_upstream_bridge()

This adds a pci_upstream_bridge() interface to find the PCI-to-PCI bridge
upstream from a device.  This is typically just "dev->bus->self", but in
the case of a VF on a virtual bus, we have to start from the corresponding
PF.  Returns NULL if there is no upstream PCI bridge, i.e., if the device
is on a root bus.

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