dmaengine: mxs-dma: fix incompatible pointer type build warns

drivers/dma/mxs-dma.c: In function 'mxs_dma_probe':
drivers/dma/mxs-dma.c:848:35: warning: assignment from incompatible pointer type [enabled by default]
drivers/dma/mxs-dma.c:849:36: warning: assignment from incompatible pointer type [enabled by default]

The function prototype expects return type 'int' whereas these where void

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