iio: adc: stm32-dfsdm: continuous mode depends on current mode

DFSDM regular continuous mode usage depends on current mode (not DMA):
- for single conversion, RCONT doesn't need to be set.
- for buffer mode, RCONT has to be set (e.g. INDIO_BUFFER_SOFTWARE
  used by audio currently).
This is related to filter configuration, move it to relevant routine.

This is precursor patch to ease support of triggered buffer mode.

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