PCI: convert pci_scan_bus_parented() to use pci_create_root_bus()

This converts pci_scan_bus_parented() to use pci_create_root_bus()
instead of pci_create_bus().  The new bus still has the default (incorrect)
resources, so this patch doesn't help fix that problem, but it does remove
one more use of pci_create_bus().

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