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>

drivers/dma/dma-jz4780.c

@@ -396,7 +396,7 @@ static struct dma_async_tx_descriptor *jz4780_dma_prep_dma_cyclic(
 	return vchan_tx_prep(&jzchan->vchan, &desc->vdesc, flags);
 }
 
-struct dma_async_tx_descriptor *jz4780_dma_prep_dma_memcpy(
+static struct dma_async_tx_descriptor *jz4780_dma_prep_dma_memcpy(
 	struct dma_chan *chan, dma_addr_t dest, dma_addr_t src,
 	size_t len, unsigned long flags)
 {