OMAP: nokia770: remove custom implementation of ads7846_get_pendown_state

The default implementation matches exactly our custom one so we can switch
to using the default one. As a bonus the driver will take care of setting
GPIO line for us.

Tested-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>