PCI/portdrv: Encapsulate pcie_ports_auto inside the port driver

"pcie_ports_auto" is only used inside the PCIe port driver itself, so
move it from include/linux/pci.h to portdrv.h so it's not visible to the
whole kernel.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>