mctp: Fix check for dev_hard_header() result

dev_hard_header() returns the length of the header, so
we need to test for negative errors rather than non-zero.

Fixes: 889b7da2 ("mctp: Add initial routing framework")
Signed-off-by: Matt Johnston <matt@codeconstruct.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mctp/route.c

@@ -512,7 +512,7 @@ static int mctp_route_output(struct mctp_route *route, struct sk_buff *skb)
 
 	rc = dev_hard_header(skb, skb->dev, ntohs(skb->protocol),
 			     daddr, skb->dev->dev_addr, skb->len);
-	if (rc) {
+	if (rc < 0) {
 		kfree_skb(skb);
 		return -EHOSTUNREACH;
 	}