dma: mxs-dma: Staticize mxs_dma_xlate

Fix the following sparse warning:

drivers/dma/mxs-dma.c:696:17: warning: symbol 'mxs_dma_xlate' was not declared. Should it be static?

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>