ethernet: ensure EthernetOutcomeReceiver receives only one result

Sending multiple callbacks to one receiver is an easy trap to fall into.
Asserting that the future has not completed in EthernetOutcomeReceiver
should catch this mistake.

Test: atest EthernetManagerTest
Change-Id: Iaaace0e6604558bfc2fc1ae58c2744185e0b13ca
1 file changed