PCI: Unexport pcie_port_bus_type

pcie_port_bus_type is used only in pci-driver.c and pcie/portdrv_core.c and
pcie/portdrv_pci.c.  None of these can be built as modules, so
pcie_port_bus_type doesn't need to be exported.  Unexport it.

Link: https://lore.kernel.org/r/20230824193712.542167-3-helgaas@kernel.org


Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>