dmaengine: ste_dma40: minor code readability fixes

Use internal variables to the cycles to improve code readability, no
functional changes.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Fabio Baltieri <fabio.baltieri@linaro.org>