dmaengine: sun6i-dma: Only calculate residue if state exists.

There is no point in calculating the residue if state does not
exist to store the value.
Signed-off-by: NPeter Griffin <peter.griffin@linaro.org>
Signed-off-by: NVinod Koul <vinod.koul@intel.com>

drivers/dma/sun6i-dma.c

@@ -865,7 +865,7 @@ static enum dma_status sun6i_dma_tx_status(struct dma_chan *chan,
 	size_t bytes = 0;
 
 	ret = dma_cookie_status(chan, cookie, state);
-	if (ret == DMA_COMPLETE)
+	if (ret == DMA_COMPLETE || !state)
 		return ret;
 
 	spin_lock_irqsave(&vchan->vc.lock, flags);