udp: fix byte order in sockaddr6_t

udp_recvfrom wrote the sender port number in host byte order into the
provided sockaddr6_t. Because all send functions expect the port number
in network byte order this introduces a superfluous conversion step in
case one wants to reuse the address for replying.

closes #1406
Merge pull request #1795 from sgso/byte-order-udp-sockaddr

udp: fix byte order in sockaddr6_t