serial: core: make serial_base_bus_type const

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.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>