ARM: mxs-dma: reset after disable channel

We met some channels in abnormal state after disable.
Reset it to get a clean state.

Signed-off-by: Dong Aisheng <b29396@freescale.com>
Cc: Vinod Koul <vinod.koul@intel.com>
Cc: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>