serial: core: make serial_base_bus_type const

Now that the driver core can properly handle constant struct bus_type,
move the serial_base_bus_type variable to be a constant structure as well,
placing it into read-only memory which can not be modified at runtime.
Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: "Ricardo B. Marliere" <ricardo@marliere.net>
Link: https://lore.kernel.org/r/20240203-bus_cleanup-tty-v1-2-86b698c82efe@marliere.netSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/serial_base_bus.c

@@ -41,7 +41,7 @@ static int serial_base_match(struct device *dev, struct device_driver *drv)
 	return 0;
 }
 
-static struct bus_type serial_base_bus_type = {
+static const struct bus_type serial_base_bus_type = {
 	.name = "serial-base",
 	.match = serial_base_match,
 };