iio: adc: stm32-dfsdm: fix clock source selection

Add missing clock source selection. In case "audio" clock is provided,
it's unused currently: "dfsdm" clock is wrongly used by default.

Fixes: bed73904 ("IIO: ADC: add stm32 DFSDM core support")

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Acked-by: Arnaud Pouliquen <arnaud.pouliquen@st.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>