iio: adc: stm32: fix vrefint wrong calibration value handling

iio: adc: stm32: fix vrefint wrong calibration value handling

If the vrefint calibration is zero, the vrefint channel output value
cannot be computed. Currently, in such case, the raw conversion value
is returned, which is not relevant.
Do not expose the vrefint channel when the output value cannot be
computed, instead.

Fixes: 0e346b2cfa85 ("iio: adc: stm32-adc: add vrefint calibration support")
Signed-off-by: Olivier Moysan <olivier.moysan@foss.st.com>
Reviewed-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Link: https://lore.kernel.org/r/20220609095856.376961-1-olivier.moysan@foss.st.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>