iio: buffer-dmaengine: Use dma_request_chan() directly for channel request

dma_request_slave_channel_reason() is:
	dma_request_chan(dev, name)

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Reviewed-by: Vinod Koul <vkoul@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>