PCI: convert pci_scan_bus() to use pci_create_root_bus()

I plan to deprecate pci_scan_bus_parented(), so use pci_create_root_bus()
directly instead.  pci_scan_bus() itself will be removed as soon as all
callers are gone, so this is just an interim step.

v2: export pci_scan_bus

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