Input: of_touchscreen - fix setting max values on X/Y axis

The binding specification says that "touchscreen-size-x" and "-y" specify
horizontal and vertical resolution of the touchscreen and therefore maximum
absolute coordinates should be reduced by 1 since we are starting with 0.

Reviewed-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>