Avoid double call to set ofono's online property

On initialization a double call to set-soft was being done for the ofono device. This caused bugs LP #1392397 and #1379807.
set_soft is only called from the arbitrator. When ofono manager detects the removal of modem, it removes the modem from the arbitrator, so I do not see the possibility of spurious calls. And the already pending callbacks will be called with a disconnection error immediately.