dmaengine: zx: fix build warning

Fix build warning that related to PAGE_SIZE. The maximum DMA
length has nothing to do with PAGE_SIZE, just use a fix number
for the definition.

drivers/dma/zx_dma.c: In function 'zx_dma_prep_memcpy':
drivers/dma/zx_dma.c:523:8: warning: division by zero [-Wdiv-by-zero]
drivers/dma/zx_dma.c: In function 'zx_dma_prep_slave_sg':
drivers/dma/zx_dma.c:567:11: warning: division by zero [-Wdiv-by-zero]

Signed-off-by: Jun Nie <jun.nie@linaro.org>
Tested-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>