destiny: fix byte order in sockaddr6_t

destiny_socket_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

I wasn't able to find any applications in the source tree that depend on this behavior, so just 1 commit.