IIO: ADC: stm32-dfsdm: fix static check warning

iio_priv does not return an error pointer, so check is not valid.
Patch suppresses it.

Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@st.com>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Mark Brown <broonie@kernel.org>