iio: adc: at91-sama5d2_adc: Use dma_request_chan() instead dma_request_slave_channel()

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

The dma_request_chan() is the standard API to request slave channel,
clients should be moved away from the legacy API to allow us to retire
them.
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/at91-sama5d2_adc.c

@@ -1444,10 +1444,10 @@ static void at91_adc_dma_init(struct platform_device *pdev)
 	if (st->dma_st.dma_chan)
 		return;
 
-	st->dma_st.dma_chan = dma_request_slave_channel(&pdev->dev, "rx");
-
-	if (!st->dma_st.dma_chan)  {
+	st->dma_st.dma_chan = dma_request_chan(&pdev->dev, "rx");
+	if (IS_ERR(st->dma_st.dma_chan))  {
 		dev_info(&pdev->dev, "can't get DMA channel\n");
+		st->dma_st.dma_chan = NULL;
 		goto dma_exit;
 	}