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.
Merge pull request #94 from alfonsosanchezbeato/mtk-flightmode

mtk: Fix race condition when onlining
rilmodem: Notify SIM removal
Merge pull request #99 from marttipiirainen/master

rilmodem: Notify SIM removal