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]