x86/PCI: use pci_scan_bus() instead of pci_scan_bus_parented()

This doesn't change any functionality, but it makes a subsequent patch
slightly simpler.

pci_scan_bus(NULL, ...) and pci_scan_bus_parented() are identical except
that pci_scan_bus() also calls pci_bus_add_devices():

  pci_scan_bus_parented
    pci_create_bus
    pci_scan_child_bus

  pci_scan_bus
    pci_create_bus
    pci_scan_child_bus
    pci_bus_add_devices

All callers of pcibios_scan_root() call pci_bus_add_devices() explicitly,
and we don't pass a parent device, so we might as well use pci_scan_bus().

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>