PCI: designware: Use pci_scan_root_bus() for simplicity

After b97ea289cf6a ("PCI: Assign resources before drivers claim devices
(pci_scan_root_bus())"), pci_scan_root_bus() no longer adds the devices, so
it is equivalent to:

pci_create_root_bus()
pci_scan_child_bus()

Use pci_scan_root_bus() to simplify the code.

[bhelgaas: changelog]
Signed-off-by: Yijing Wang <[e-mail]>
Signed-off-by: Bjorn Helgaas <[e-mail]>
Reviewed-by: Lucas Stach <[e-mail]>
Acked-by: Jingoo Han <[e-mail]>
CC: Mohit Kumar <[e-mail]>

drivers/pci/host/pcie-designware.c

···
 	struct pcie_port *pp = sys_to_pcie(sys);
 
 	pp->root_bus_nr = sys->busnr;
-	bus = pci_create_root_bus(pp->dev, sys->busnr,
+	bus = pci_scan_root_bus(pp->dev, sys->busnr,
 				  &dw_pcie_ops, sys, &sys->resources);
 	if (!bus)
 		return NULL;
-
-	pci_scan_child_bus(bus);
 
 	if (bus && pp->ops->scan_bus)
 		pp->ops->scan_bus(pp);