Input: gpio_mouse - rename platform data variables

Use more appropriate names for the "platform data" which is now just a
simple state container for the GPIO mouse.

Acked-by: Hans-Christian Noren Egtvedt <egtvedt@samfundet.no>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>