drivers/cst816s: use gpio_is_valid where appropriate
drivers/cst816s: use gpio_is_valid where appropriate

kaspar030/RIOT,jasonatran/RIOT,miri64/RIOT,RIOT-OS/RIOT,miri64/RIOT,jasonatran/RIOT,ant9000/RIOT,OlegHahm/RIOT,kaspar030/RIOT,kaspar030/RIOT,ant9000/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,miri64/RIOT,kaspar030/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,ant9000/RIOT,jasonatran/RIOT,kYc0o/RIOT,miri64/RIOT,OlegHahm/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,kaspar030/RIOT,miri64/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,jasonatran/RIOT,kYc0o/RIOT,ant9000/RIOT,ant9000/RIOT