ia64/PCI: use pci_create_bus() instead of pci_scan_bus_parented()

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

CC: Tony Luck <tony.luck@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>