dmaengine: k3dma: fix double free of descriptor

Commit 36387a2b ("k3dma: Fix
memory handling in preparation for cyclic mode") adds code
to free the descriptor in ds_done.

In cyclic mode, ds_done is never used and it's always NULL,
so the added code is not executed.

In non-cyclic mode, ds_done is used as a flag: when not NULL
it signals that the descriptor has been consumed. No need to
free it because it would be free by vchan_complete().

The fix takes back the code changed by the commit above:
- remove the free on the descriptor;
- initialize ds_done to NULL for the next run.

Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>