iio: adc: stm32-adc: dma transfers cleanup

- Remove processing related to DMA in irq handler as this
data transfer is managed directly in DMA callback.
- Update comment in stm32_adc_set_watermark() function.

Signed-off-by: Olivier Moysan <olivier.moysan@st.com>
Reviewed-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Link: https://lore.kernel.org/r/20201105142941.27301-1-olivier.moysan@st.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>