tilegx pci: fix semantic merge conflict with 3527ed81

Yinghai Lu removed pci_bus.subordinate in pci-next, which meant that
the tile-next changes to add tilegx PCI support don't build.  This
was expected (seen in linux-next) and this one-line fix is along
the same lines as commit b918c62e for all other architectures.

Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>