ethernet: fix enable/disableInterface result

Calling enableInterface() / disableInterface() for an interface already
in the correct state used to return an error which is incorrect
according to the API specification in EthernetManager. onError() is now
only returned for untracked and server mode interfaces.

This change also fixes the updateInterfaceState() function signature.

Test: atest EthernetManagerTest
Change-Id: Ie7ee84c3756a8a0c489a38548214fd524352c8da
2 files changed