x86/PCI: Drop pcibios_scan_root() check for bus already scanned

The PCI core checks to see whether we've already scanned a bus, so we don't
need to do it in pcibios_scan_root().  Here's where it happens in the core:

  pcibios_scan_root
    pci_scan_bus_on_node
      pci_scan_root_bus
        pci_create_root_bus
	  b2 = pci_find_bus(pci_domain_nr(b), bus)
	  if (b2)
	    goto err_out;    # already scanned this bus

This removes the check from pcibios_scan_root().

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