dmaengine: sprd: statify 'sprd_dma_prep_dma_memcpy'

Sparse warns that 'sprd_dma_prep_dma_memcpy' should be static so make it
static.

drivers/dma/sprd-dma.c:713:32: warning:
symbol'sprd_dma_prep_dma_memcpy' was not declared. Should it be static?
Reviewed-by: NBaolin Wang <baolin.wang@linaro.org>
Signed-off-by: NVinod Koul <vinod.koul@intel.com>

drivers/dma/sprd-dma.c

@@ -710,7 +710,7 @@ static int sprd_dma_config(struct dma_chan *chan, struct sprd_dma_desc *sdesc,
 	return 0;
 }
 
-struct dma_async_tx_descriptor *
+static struct dma_async_tx_descriptor *
 sprd_dma_prep_dma_memcpy(struct dma_chan *chan, dma_addr_t dest, dma_addr_t src,
 			 size_t len, unsigned long flags)
 {