Multiple calls to Drv_EddRegisterEthCb() with same ethertype

If Drv_EddRegisterEthCb() is called with an ethertype parameter value which has already been registered by a previous call to the function, then the function succeeds and registers the callback as requested.

However, when received frames are dispatched among the registered callbacks, then the first match wins and will be dispatched the frame, so the callback registered later in time is useless.

It pretty much would make sense to reject a callback registration for an already registered ethertype with a meaningful error code.