mailbox: bcm-ferxrm-mailbox: Fix error check for dma_map_sg

dma_map_sg return 0 on error, fix the error check, and return -EIO
to caller.

Fixes: dbc049eee7 ("mailbox: Add driver for Broadcom FlexRM ring manager")
Signed-off-by: Jack Wang <jinpu.wang@ionos.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

drivers/mailbox/bcm-flexrm-mailbox.c

@@ -622,15 +622,15 @@ static int flexrm_spu_dma_map(struct device *dev, struct brcm_message *msg)
 
 	rc = dma_map_sg(dev, msg->spu.src, sg_nents(msg->spu.src),
 			DMA_TO_DEVICE);
-	if (rc < 0)
-		return rc;
+	if (!rc)
+		return -EIO;
 
 	rc = dma_map_sg(dev, msg->spu.dst, sg_nents(msg->spu.dst),
 			DMA_FROM_DEVICE);
-	if (rc < 0) {
+	if (!rc) {
 		dma_unmap_sg(dev, msg->spu.src, sg_nents(msg->spu.src),
 			     DMA_TO_DEVICE);
-		return rc;
+		return -EIO;
 	}
 
 	return 0;