6lowpan: fix error checking code

Bool variable 'fail' is always non-negative, it indicates an error if it
is true.

The problem has been detected using coccinelle script
scripts/coccinelle/tests/unsigned_lesser_than_zero.cocci
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Acked-by: Alexander Aring <aar@pengutronix.de>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

net/6lowpan/iphc.c

@@ -482,7 +482,7 @@ static int lowpan_uncompress_multicast_ctx_daddr(struct sk_buff *skb,
 	ipaddr->s6_addr[0] = 0xFF;
 	fail = lowpan_fetch_skb(skb, &ipaddr->s6_addr[1], 2);
 	fail |= lowpan_fetch_skb(skb, &ipaddr->s6_addr[12], 4);
-	if (fail < 0)
+	if (fail)
 		return -EIO;
 
 	/* take prefix_len and network prefix from the context */