Input: serio - make serio_bus const

Now that the driver core can properly handle constant struct bus_type,
move the serio_bus 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/20240210-bus_cleanup-input2-v1-2-0daef7e034e0@marliere.net
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>