x86/PCI: Merge pci_scan_bus_on_node() into pcibios_scan_root()

pci_scan_bus_on_node() is only called by pcibios_scan_root().
This merges pci_scan_bus_on_node() into pcibios_scan_root() and removes
pci_scan_bus_on_node().

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