ieee802154: 6lowpan: change datagram var types

This patch changes datagram size variable from u16 type to unsigned int.
The reason is that an IPv6 header has an MAX_UIN16 payload length, but
the datagram size is payload + IPv6 header length. This avoids overflows
at some places.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>