dmaengine: jz4740: Fix non-cyclic descriptor completion

We need to make sure to deqeueue the descriptor from the active list before
we call vchan_cookie_complete(). Also we need obviously only set chan->desc
to NULL after we stopped using it.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>