6lowpan: fix udp compress ordering

In case ((ntohs(uh->source) & LOWPAN_NHC_UDP_8BIT_MASK) the order of
uncompression is wrong. It's always first source port then destination
port as second.

See:
http://tools.ietf.org/html/rfc6282#section-4.3.3



"Fields carried in-line (in part or in whole) appear in the same order
as they do in the UDP header format"

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