dmaengine: doc: client: Update for dmaengine_get_dma_device() usage

Client drivers should use the dmaengine_get_dma_device(chan) to get the
device pointer which should be used for DMA API for allocations and
mapping.
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Link: https://lore.kernel.org/r/20201208090440.31792-10-peter.ujfalusi@ti.comSigned-off-by: Vinod Koul <vkoul@kernel.org>

Documentation/driver-api/dmaengine/client.rst

@@ -120,7 +120,9 @@ The details of these operations are:
 
   .. code-block:: c
 
-     nr_sg = dma_map_sg(chan->device->dev, sgl, sg_len);
+     struct device *dma_dev = dmaengine_get_dma_device(chan);
+
+     nr_sg = dma_map_sg(dma_dev, sgl, sg_len);
 	if (nr_sg == 0)
 		/* error */