We used to add the _PRT after enumerating devices behind a new host
bridge.  This moves the _PRT addition *before* the enumeration, since
it no longer depends on the struct pci_bus existing.  This is one
step towards consolidating the .add/.start methods.
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Taku Izumi <izumim.taku@jp.fujitsu.com>