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

drivers/dma/mmp_tdma.c

@@ -427,6 +427,11 @@ static struct dma_async_tx_descriptor *mmp_tdma_prep_dma_cyclic(
 	int num_periods = buf_len / period_len;
 	int i = 0, buf = 0;
 
+	if (!is_slave_direction(direction)) {
+		dev_err(tdmac->dev, "unsupported transfer direction\n");
+		return NULL;
+	}
+
 	if (tdmac->status != DMA_COMPLETE) {
 		dev_err(tdmac->dev, "controller busy");
 		return NULL;