PCI: fix wrong assumption in pci_find_upstream_pcie_bridge

Current pci_find_upstream_pcie_bridge() has a wrong assumption that
pci_bus->self is NULL on the root pci bus. But it might not true on
some platforms. Because of this wrong assumption, current
pci_find_upstream_pcie_bridge() might cause endless loop. We must
check pci_bus->parent instead.

Signed-off-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>