iio: adc: stm32-dfsdm: adapt sampling rate to oversampling ratio

Update sampling rate when oversampling ratio is changed
through the IIO ABI.

Signed-off-by: Olivier Moysan <olivier.moysan@st.com>
Acked-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>