dma: fix build warnings in txx9

The unmap rework missed this:
drivers/dma/txx9dmac.c:409:25: warning: unused variable 'ds' [-Wunused-variable]

Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>