iio: adc: stm32-dfsdm: move dma slave config to start routine

Move DMA slave configuration to start routine: depending on regular or
injected mode is in use, DMA needs to read resp. RDATAR or JDATAR.
This is precursor patch to introduce injected mode (used for scan).

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