iio: adc: stm32-dfsdm: enable hw consumer

Optionally enable IIO hw consumer, when provided (e.g. for DFSDM_IIO type).
This is precursor patch to introduce buffer modes.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>