iio: srf04: fix wrong limitation in distance measuring

The measured time value in the driver is limited to the maximum distance
which can be read by the sensor. This limitation was wrong and is fixed
by this patch.

It also takes into account that we are supporting a variety of sensors
today and that the recently added sensors have a higher maximum
distance range.

Changes in v2:
- Added a Tested-by

Suggested-by: Zbyněk Kocur <zbynek.kocur@fel.cvut.cz>
Tested-by: Zbyněk Kocur <zbynek.kocur@fel.cvut.cz>
Signed-off-by: Andreas Klinger <ak@it-klinger.de>
Cc:<Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>