dmaengine: ti: edma: fix null dereference because of a typo in pointer name

Currently there is a dereference of the null pointer m_ddev.  This appears
to be a typo on the pointer, I believe s_ddev should be used instead.
Fix this by using the correct pointer.

Fixes: eb0249d5

 ("dmaengine: ti: edma: Support for interleaved mem to mem transfer")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Addresses-Coverity: ("Explicit null dereferenced")
Link: https://lore.kernel.org/r/20200226185921.351693-1-colin.king@canonical.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>