dmaengine: fix - error: potential NULL dereference 'chan'

commit 7bb587f4 "dmaengine: add interface of dma_get_slave_channel" introduced
the above error so fix it

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Suggested-by: Zhangfei Gao <zhangfei.gao@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>

drivers/dma/dmaengine.c

@@ -514,16 +514,16 @@ struct dma_chan *dma_get_slave_channel(struct dma_chan *chan)
 	/* lock against __dma_request_channel */
 	mutex_lock(&dma_list_mutex);
 
-	if (chan->client_count == 0)
+	if (chan->client_count == 0) {
 		err = dma_chan_get(chan);
-	else
+		if (err)
+			pr_debug("%s: failed to get %s: (%d)\n",
+				__func__, dma_chan_name(chan), err);
+	} else
 		chan = NULL;
 
 	mutex_unlock(&dma_list_mutex);
 
-	if (err)
-		pr_debug("%s: failed to get %s: (%d)\n",
-			__func__, dma_chan_name(chan), err);
 
 	return chan;
 }