iio: srf04: fix wrong limitation in distance measuring

	udelay(data->cfg->trigger_pulse_us);

	gpiod_set_value(data->gpiod_trig, 0);

	/* it cannot take more than 20 ms */

	/* it should not take more than 20 ms until echo is rising */

	ret = wait_for_completion_killable_timeout(&data->rising, HZ/50);

	if (ret < 0) {

		mutex_unlock(&data->lock);

		return -ETIMEDOUT;

	}

	ret = wait_for_completion_killable_timeout(&data->falling, HZ/50);

	/* it cannot take more than 50 ms until echo is falling */

	ret = wait_for_completion_killable_timeout(&data->falling, HZ/20);

	if (ret < 0) {

		mutex_unlock(&data->lock);

		return ret;

	dt_ns = ktime_to_ns(ktime_dt);

	/*

	 * measuring more than 3 meters is beyond the capabilities of

	 * the sensor

	 * measuring more than 6,45 meters is beyond the capabilities of

	 * the supported sensors

	 * ==> filter out invalid results for not measuring echos of

	 *     another us sensor

	 *

	 * formula:

	 *         distance       3 m

	 * time = ---------- = --------- = 9404389 ns

	 *         distance     6,45 * 2 m

	 * time = ---------- = ------------ = 40438871 ns

	 *          speed         319 m/s

	 *

	 * using a minimum speed at -20 °C of 319 m/s

	 */

	if (dt_ns > 9404389)

	if (dt_ns > 40438871)

		return -EIO;

	time_ns = dt_ns;

	 *   with Temp in °C

	 *   and speed in m/s

	 *

	 * use 343 m/s as ultrasonic speed at 20 °C here in absence of the

	 * use 343,5 m/s as ultrasonic speed at 20 °C here in absence of the

	 * temperature

	 *

	 * therefore:

	 *             time     343

	 * distance = ------ * -----

	 *             10^6       2

	 *             time     343,5     time * 106

	 * distance = ------ * ------- = ------------

	 *             10^6         2         617176

	 *   with time in ns

	 *   and distance in mm (one way)

	 *

	 * because we limit to 3 meters the multiplication with 343 just

	 * because we limit to 6,45 meters the multiplication with 106 just

	 * fits into 32 bit

	 */

	distance_mm = time_ns * 343 / 2000000;

	distance_mm = time_ns * 106 / 617176;

	return distance_mm;

}