6lowpan: fix udp header compression when using raw sockets

If you use RAW sockets the transport header offset is not set by the
ipv6 stack so when we get to the udp header compression it does not
compress the right part of the packet.

This patch adds a check for this scenario and sets the transport
header offset.

Signed-off-by: Simon Vincent <simon.vincent@xsilon.com>
Acked-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>