dmaengine: ste_dma40: Only calculate residue if txstate exists.

There is no point calculating the residue if there is
no txstate to store the value.
Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>

drivers/dma/ste_dma40.c

@@ -2588,7 +2588,7 @@ static enum dma_status d40_tx_status(struct dma_chan *chan,
 	}
 
 	ret = dma_cookie_status(chan, cookie, txstate);
-	if (ret != DMA_COMPLETE)
+	if (ret != DMA_COMPLETE && txstate)
 		dma_set_residue(txstate, stedma40_residue(chan));
 
 	if (d40_is_paused(d40c))