x86/PCI: Use pcibios_scan_root() instead of pci_scan_bus_on_node()

pcibios_scan_root() looks up the bus's NUMA node, then calls
pci_scan_bus_on_node().  This uses pcibios_scan_root() directly and drops
the node lookup in the callers.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>