mtd/nand: Fix denali build on ppc64

drivers/mtd/nand/denali.c:1427: error: conflicting types for ‘enable_dma’
arch/powerpc/include/asm/dma.h:189: note: previous definition of ‘enable_dma’ was here

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>