dmaengine: fsl-edma: 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

Tested-by: Krzysztof Kozlowski <krzk@kernel.org>
Tested-by: Angelo Dureghello <angelo@sysam.it>
Signed-off-by: Vinod Koul <vkoul@kernel.org>