dmaengine: jz4780: make jz4780_dma_prep_dma_memcpy static

Sparse complains:
drivers/dma/dma-jz4780.c:399:32: warning: symbol
'jz4780_dma_prep_dma_memcpy' was not declared. Should it be static?

So make this static

Signed-off-by: Vinod Koul <vinod.koul@intel.com>