dma: mxs-dma: Fix build warnings with W=1

Fix the following warnings when building with W=1 option:

drivers/dma/mxs-dma.c: In function 'mxs_dma_alloc_chan_resources':
drivers/dma/mxs-dma.c:368:25: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
drivers/dma/mxs-dma.c: In function 'mxs_dma_prep_slave_sg':
drivers/dma/mxs-dma.c:481:17: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
drivers/dma/mxs-dma.c:494:3: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
drivers/dma/mxs-dma.c:515:14: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
drivers/dma/mxs-dma.c: In function 'mxs_dma_prep_dma_cyclic':
drivers/dma/mxs-dma.c:563:13: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>