rilmodem: Fix crash when switching on/off FM

Due to a race condition, we can crash if we go too fast from FM on
to FM off and then back fast to on. This is due to a pending callback
that was not removed from the main loop on the removal of the gprs atom.
Merge pull request #124 from alfonsosanchezbeato/fix-fm-crash

rilmodem: Fix crash when switching on/off FM