dmaengine: mmp_tdma: Validate the transfer direction

We only support DMA_DEV_TO_MEM and DMA_MEM_TO_DEV. Let's not do
undefined things with other values and reject them.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Link: https://lore.kernel.org/r/20200424215020.105281-1-lkundrak@v3.sk


Signed-off-by: Vinod Koul <vkoul@kernel.org>