drivers/rapidio/devices/rio_mport_cdev.c: fix error handling in 'rio_dma_transfer()'

In case of error, 'dma_map_sg()' returns 0, not a negative value.  There
is BUG_ON() in 'dma_map_sg_attrs()' which makes sure of that.

Link: http://lkml.kernel.org/r/d4235bd2b9274e99f6c86ea71b1fa1c7bd8d0c08.1505687047.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Cc: Matt Porter <mporter@kernel.crashing.org>
Cc: Alexandre Bounine <alexandre.bounine@idt.com>
Cc: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: Jesper Nilsson <jesper.nilsson@axis.com>
Cc: Christian K_nig <christian.koenig@amd.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/rapidio/devices/rio_mport_cdev.c

@@ -961,7 +961,7 @@ rio_dma_transfer(struct file *filp, u32 transfer_mode,
 
 	nents = dma_map_sg(chan->device->dev,
 			   req->sgt.sgl, req->sgt.nents, dir);
-	if (nents == -EFAULT) {
+	if (nents == 0) {
 		rmcd_error("Failed to map SG list");
 		ret = -EFAULT;
 		goto err_pg;