ieee802154: 6lowpan: fix drop return value

This patch changes the return value of lowpan packet receive handler to
the correct NET_RX_DROP instead RX_DROP.

This issue was detected by sparse and reported from Marcel:
net/ieee802154/6lowpan/rx.c:329:32:    expected int
net/ieee802154/6lowpan/rx.c:329:32:    got restricted lowpan_rx_result ...
Reported-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

net/ieee802154/6lowpan/rx.c

@@ -326,7 +326,7 @@ static int lowpan_rcv(struct sk_buff *skb, struct net_device *wdev,
 	    lowpan_is_iphc(*skb_network_header(skb))) {
 		skb = skb_unshare(skb, GFP_ATOMIC);
 		if (!skb)
-			return RX_DROP;
+			return NET_RX_DROP;
 	}
 
 	return lowpan_invoke_rx_handlers(skb);