dmaengine/ste_dma40: fix introduced warnings

The compiler nowadays moans about possibly non-assigned variable.
Fix this by default-assigning 0.

Signed-off-by: Philippe Langlais <philippe.langlais@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>