iio: bmp280: fix compensation of humidity

iio: bmp280: fix compensation of humidity

Limit the output of humidity compensation to the range between 0 and 100
percent.

Depending on the calibration parameters of the individual sensor it
happens, that a humidity above 100 percent or below 0 percent is
calculated, which don't make sense in terms of relative humidity.

Add a clamp to the compensation formula as described in the datasheet of
the sensor in chapter 4.2.3.

Although this clamp is documented, it was never in the driver of the
kernel.

It depends on the circumstances (calibration parameters, temperature,
humidity) if one can see a value above 100 percent without the clamp.
The writer of this patch was working with this type of sensor without
noting this error. So it seems to be a rare event when this bug occures.

Signed-off-by: Andreas Klinger <ak@it-klinger.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>