dmaengine: zxdma: Fix force stop bug

DMA will not stop when clearing enable bit till all transaction
is done. The bug is exposed in audio playback because ring DMA
chain never stop. Force hardware to stop with setting FORCE bit.

Signed-off-by: Jun Nie <jun.nie@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>