ethernet: fix updateConfiguration expectations

Since the mainline module is required to pass CTS, expectations that
test implementation details need to be removed from tests. For
updateConfiguration specifically, this means removing the expectLost() /
expectAvailable() expectation that is a symptom of the current
implementation but does not affect that actual outcome of this API.

Test: atest EthernetManagerTest --iterations 5
Change-Id: I40c38a1fe847eb4166e75535cf70db9ba569bc43