emb6_sock_udp: copy receive remote correctly

The `src` member of `sock_udp_t` for `emb6` is a pointer, not a value,
so it should not be referenced.

This fixes the output issue encountered during the
[2018.10 RC1 testing][RC1].

[RC1]: https://github.com/RIOT-OS/Release-Specs/issues/76#issuecomment-435924505