dmaengine: ste_dma40: make stedma40_chan_cfg const

Make these const as they are only used during a copy operation.
Done using Coccinelle.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>