OFDDPSocket: Don't include the type with the data

This seems to be an oddity limited to OSes that have implemented DDP
exclusively for netatalk, while macOS and Windows don't include it with
the data.

While on macOS it's possible to achieve the previous behavior via some
hacks, this is impossible on Windows, so the proper approach is to
handle it like everybody else: Specify the protocol type when binding
and only handle packets of the correct protocol type.