PCI: Make pcie_port_bus_type const

PCI: Make pcie_port_bus_type const

Now that the driver core can properly handle constant struct bus_type, move
the pcie_port_bus_type variable to be a constant structure as well, placing
it into read-only memory which can not be modified at runtime.

Link: https://lore.kernel.org/r/20240208-bus_cleanup-pci2-v1-1-5e578210b6f2@marliere.net
Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ricardo B. Marliere <ricardo@marliere.net>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>