mtk: Fix race condition when onlining

There was a race condition in case both modems were onlined almost at the same time (lp bug #1329707). The fix serializes power requests to the modem.
This should be clarified as serializing online requests sent to the two different modem instances.  Ofono normally takes care of serializing requests to the same modem, but as this code attempts to hide the details of the power on sequence from the core, this extra serialization is necessary.