ieee802154: 6lowpan: fix error frag handling

This patch fixes the error handling for lowpan_xmit_fragment by replace
"-PTR_ERR" to "PTR_ERR". PTR_ERR returns already a negative errno code.
Signed-off-by: NAlexander Aring <alex.aring@gmail.com>
Signed-off-by: NMarcel Holtmann <marcel@holtmann.org>

net/ieee802154/6lowpan/tx.c

@@ -112,7 +112,7 @@ lowpan_xmit_fragment(struct sk_buff *skb, const struct ieee802154_hdr *wpan_hdr,
 
 	frag = lowpan_alloc_frag(skb, frag_hdrlen + len, wpan_hdr);
 	if (IS_ERR(frag))
-		return -PTR_ERR(frag);
+		return PTR_ERR(frag);
 
 	memcpy(skb_put(frag, frag_hdrlen), frag_hdr, frag_hdrlen);
 	memcpy(skb_put(frag, len), skb_network_header(skb) + offset, len);