Input: make input_class constant

Since commit 43a7206b

 ("driver core: class: make class_register() take
a const *"), the driver core allows for struct class to be in read-only
memory, so move the input_class structure to be declared at build time
placing it into read-only memory, instead of having to be dynamically
allocated at boot time.

Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ricardo B. Marliere <ricardo@marliere.net>
Link: https://lore.kernel.org/r/20240305-class_cleanup-input-v1-1-0c3d950c25db@marliere.net
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>