dma: fix build warnings in ppc4xx

drivers/dma/ppc4xx/adma.c:1507:6: warning: unused variable 'i' [-Wunused-variable]
- due to unmap reworks

drivers/dma/ppc4xx/adma.c:3900:2: warning: format '%s' expects a matching 'char *' argument [-Wformat]
- due to memset removal

drivers/dma/ppc4xx/adma.c:538:13: warning: 'ppc440spe_desc_init_memset' defined but not used [-Wunused-function]
- due to memset removal

Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>