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>