ptp: Send packets on the interface that we received on

With gst-ptp-helper we send on the socket without specifying an interface, which might be any of the interfaces on the system (since there is no binding of the socket to any given interface). Even specifying an interface during init does not solve this (as that only determines the interface on which we join the multicast group, but not what we send on).

With multiple interfaces and the PTP master being on an interface that is not selected by default for sending, this may result in delay request packets never really being sent.