dmaengine: mxs-dma: fix unused variable warn

drivers/dma/mxs-dma.c: In function 'mxs_dma_terminate_all':
drivers/dma/mxs-dma.c:662:23: warning: unused variable 'mxs_chan'[-Wunused-variable]

Signed-off-by: Vinod Koul <vinod.koul@intel.com>