dmaengine: bcm2835: fix unexported function

The bcm2835_dma_prep_dma_memcpy() function is not exported
outside the driver, so make it static to avoid the following
warning:

drivers/dma/bcm2835-dma.c:616:32: warning: symbol 'bcm2835_dma_prep_dma_memcpy' was not declared. Should it be static?
Signed-off-by: NBen Dooks <ben.dooks@codethink.co.uk>
Reviewed-by: NEric Anholt <eric@anholt.net>
Signed-off-by: NVinod Koul <vinod.koul@intel.com>

drivers/dma/bcm2835-dma.c

@@ -613,7 +613,7 @@ static void bcm2835_dma_issue_pending(struct dma_chan *chan)
 	spin_unlock_irqrestore(&c->vc.lock, flags);
 }
 
-struct dma_async_tx_descriptor *bcm2835_dma_prep_dma_memcpy(
+static struct dma_async_tx_descriptor *bcm2835_dma_prep_dma_memcpy(
 	struct dma_chan *chan, dma_addr_t dst, dma_addr_t src,
 	size_t len, unsigned long flags)
 {