dmaengine: dw: 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

Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>