ieee802154: 6lowpan: ensure of sending 1280 packets

This patch changes the 1281 MTU to 1280. Others stack have only a 1280
byte array for uncompressed 6LoWPAN packets, this avoid that these
stacks have an overflow. Sending 1281 uncompressed 6LoWPAN packets isn't
also rfc complaint.
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

net/ieee802154/6lowpan_rtnl.c

@@ -437,7 +437,7 @@ static void lowpan_setup(struct net_device *dev)
 	/* Frame Control + Sequence Number + Address fields + Security Header */
 	dev->hard_header_len	= 2 + 1 + 20 + 14;
 	dev->needed_tailroom	= 2; /* FCS */
-	dev->mtu		= 1281;
+	dev->mtu		= IPV6_MIN_MTU;
 	dev->tx_queue_len	= 0;
 	dev->flags		= IFF_BROADCAST | IFF_MULTICAST;
 	dev->watchdog_timeo	= 0;