PCI: designware: Use pci_scan_root_bus() for simplicity

After b97ea289 ("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>