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>