dma: imx-sdma: Adding tasklet_kill() in sdma_remove function.

Several dma drivers calls tasklet_kill() in remove function. This is done
because all running tasklets should be killed on remove. This is missing
in imx sdma driver, so adding tasklet_kill() in sdma_remove function.

Signed-off-by: Vignesh Raman <Vignesh_Raman@mentor.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>