In an attempt to "unuse" pci_do_scan_bus() so it can be eventually
killed, make pci_scan_bus_parented() call the new pci_scan_child_bus()
and pci_bus_add_devices().  The only remaining callers are the
hotplug drivers.

Eventually, pci_bus_add_devices() will be removed from this function -
it is intended that architectures should call this after they have
done any setups and fixups to the scanned bus.

It is legal to call pci_bus_add_devices() on a bus which has already
had this function called, so architectures could update today.