Input: allow passing NULL to input_free_device()

Many drivers rely on input_free_device() behaving like kfree().
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>

include/linux/input.h

@@ -1016,6 +1016,7 @@ static inline void input_put_device(struct input_dev *dev)
 
 static inline void input_free_device(struct input_dev *dev)
 {
+	if (dev)
 		input_put_device(dev);
 }