iio: adc: stm32-dfsdm: move dma enable from start_conv() to start_dma()

Move DMA enable (e.g. set RDMAEN bit) away from start_conv() that is used
for both buffer and single conversions. Thus, single conv rely on
interrupt, not dma.
Note: take care to prepare all DMA stuff and set RDMAEN before starting
filter (can be set only when DFEN=0).

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>