net: openthread: Process real ACK frame instead of fake one

OpenThread radio layer did not implement `ieee802154_radio_handle_ack`
API and provided fake ACK frame to the OpenThread.

This prevented proper Sleepy End Device operation, as it expects to
receive information in the ACK whether it should wait for more data to
come or should it put the radio to sleep.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>