ieee802154: 6lowpan: fix sign of errno return val

This patch fix ERR_PTR(-rc) to ERR_PTR(rc). The variable rc is already a
negative errno value.
Signed-off-by: NAlexander Aring <alex.aring@gmail.com>
Signed-off-by: NMarcel Holtmann <marcel@holtmann.org>

net/ieee802154/6lowpan_rtnl.c

@@ -233,7 +233,7 @@ lowpan_alloc_frag(struct sk_buff *skb, int size,
 				     &master_hdr->source, size);
 		if (rc < 0) {
 			kfree_skb(frag);
-			return ERR_PTR(-rc);
+			return ERR_PTR(rc);
 		}
 	} else {
 		frag = ERR_PTR(-ENOMEM);