iio: adc: at91-sama5d2: Replace 0 with NULL when clearing some pointers.

Cleans up the sparse warning:

CHECK   drivers/iio/adc/at91-sama5d2_adc.c
drivers/iio/adc/at91-sama5d2_adc.c:1486:31: warning: Using plain integer as NULL pointer
drivers/iio/adc/at91-sama5d2_adc.c:1509:31: warning: Using plain integer as NULL pointer
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Eugen Hristev <eugen.hristev@microchip.com>

drivers/iio/adc/at91-sama5d2_adc.c

@@ -1483,7 +1483,7 @@ static void at91_adc_dma_init(struct platform_device *pdev)
 			  st->dma_st.rx_buf, st->dma_st.rx_dma_buf);
 dma_chan_disable:
 	dma_release_channel(st->dma_st.dma_chan);
-	st->dma_st.dma_chan = 0;
+	st->dma_st.dma_chan = NULL;
 dma_exit:
 	dev_info(&pdev->dev, "continuing without DMA support\n");
 }
@@ -1506,7 +1506,7 @@ static void at91_adc_dma_disable(struct platform_device *pdev)
 	dma_free_coherent(st->dma_st.dma_chan->device->dev, pages * PAGE_SIZE,
 			  st->dma_st.rx_buf, st->dma_st.rx_dma_buf);
 	dma_release_channel(st->dma_st.dma_chan);
-	st->dma_st.dma_chan = 0;
+	st->dma_st.dma_chan = NULL;
 
 	dev_info(&pdev->dev, "continuing without DMA support\n");
 }