dmaengine: ste_dma40: remove dma_slave_config direction usage

dma_slave_config direction was marked as deprecated quite some
time back, remove the usage from this driver so that the field
can be removed

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Vinod Koul <vkoul@kernel.org>