dma: imx-sdma: remove the unused completion

After the patch: "2ccaef05 dma: imx-sdma: make channel0 operations atomic",
the "done" completion is not used any more.

Just remove it.

Signed-off-by: Huang Shijie <b32955@freescale.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>