Input: mark some functions __must_check

Mark input_register_device() and input_register_handler() functions
as __must_check so authors of new drivers add error handling right
away.

Signed-off-by: Dmitry Torokhov <dtor@mail.ru>

include/linux/input.h

@@ -1125,10 +1125,10 @@ static inline void input_set_drvdata(struct input_dev *dev, void *data)
 	dev->private = data;
 }
 
-int input_register_device(struct input_dev *);
+int __must_check input_register_device(struct input_dev *);
 void input_unregister_device(struct input_dev *);
 
-int input_register_handler(struct input_handler *);
+int __must_check input_register_handler(struct input_handler *);
 void input_unregister_handler(struct input_handler *);
 
 int input_register_handle(struct input_handle *);