iio: adc: stm32-dfsdm: Use dma_request_chan() instead dma_request_slave_channel()

[ Upstream commit a9ab624edd9186fbad734cfe5d606a6da3ca34db ]

dma_request_slave_channel() is a wrapper on top of dma_request_chan()
eating up the error code.

By using dma_request_chan() directly the driver can support deferred
probing against DMA.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Acked-by: Olivier Moysan <olivier.moysan@st.com>
Acked-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>