Input: gpio_mouse - switch to using input device polling mode

Now that instances of input_dev support polling mode natively,
we no longer need to create input_polled_dev instance.

Acked-by: Hans-Christian Noren Egtvedt <egtvedt@samfundet.no>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>