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