PCI: designware: Use pci_scan_root_bus() for simplicity

After b97ea28 ("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 <wangyijing@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Acked-by: Jingoo Han <jingoohan1@gmail.com>
CC: Mohit Kumar <mohit.kumar@st.com>

drivers/pci/host/pcie-designware.c

@@ -728,13 +728,11 @@ static struct pci_bus *dw_pcie_scan_bus(int nr, struct pci_sys_data *sys)
 	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);