tile/PCI: use pci_scan_root_bus instead pci_scan_bus

It will update busn_res accordingly, so we get that for last_busno.

Acked-by: Chris Metcalf <cmetcalf@tilera.com>
Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>