bcm2835-dma: Fix dreq not set for slave transfers

Set dreq to slave_id if it is not set like in bcm2708-dmaengine.

Author: HiassofT

drivers/dma/bcm2835-dma.c

@@ -657,6 +657,8 @@ static int bcm2835_dma_slave_config(struct dma_chan *chan,
 	}
 
 	c->cfg = *cfg;
+	if (!c->dreq)
+		c->dreq = cfg->slave_id;
 
 	return 0;
 }