virtio: net: make it clear that virtqueue_add_buf() no longer returns > 0

We simplified virtqueue_add_buf(), make it clear in the callers.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 7c7f5a9..6289891 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -635,7 +635,7 @@
 	err = xmit_skb(vi, skb);
 
 	/* This should not happen! */
-	if (unlikely(err < 0)) {
+	if (unlikely(err)) {
 		dev->stats.tx_fifo_errors++;
 		if (net_ratelimit())
 			dev_warn(&dev->dev,