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>

drivers/pci/pci-driver.c

@@ -1714,7 +1714,7 @@ static int pcie_port_bus_match(struct device *dev, struct device_driver *drv)
 	return 1;
 }
 
-struct bus_type pcie_port_bus_type = {
+const struct bus_type pcie_port_bus_type = {
 	.name		= "pci_express",
 	.match		= pcie_port_bus_match,
 };

drivers/pci/pcie/portdrv.h

@@ -96,7 +96,7 @@ struct pcie_port_service_driver {
 int pcie_port_service_register(struct pcie_port_service_driver *new);
 void pcie_port_service_unregister(struct pcie_port_service_driver *new);
 
-extern struct bus_type pcie_port_bus_type;
+extern const struct bus_type pcie_port_bus_type;
 
 struct pci_dev;