device: don't transition assumed devices to FAILED before ACTIVATED
If the activation of an assumed device fails, we first set the device state to FAILED and then to ACTIVATED. In the FAILED state, the active connection transitions to DEACTIVATED and clears its device pointer; hence we end up with an inconsistent state which causes assertion failures in other parts of the code (for example, get_best_ip_config() assumes that the device of the best active connection is not NULL).
Don't first transition to FAILED and then to ACTIVATED, just set the latter.