Input: wacom - make LED status readable through sysfs

Reviewed-by: Eduard Hasenleithner <eduard@hasenleithner.at>
Tested-by: Eduard Hasenleithner <eduard@hasenleithner.at>
Signed-off-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>