N900 sensors cannot run slowly

The N900 code forces frequent polling of the sensors even if the application does not need/want this. The code should probably do what the MeeGo backend does, allowing from 1 to max rate instead of forcing a specific rate. The underlying hardware won't change speed but the polling loop will run less frequently.

Related to this, the proximity sensor runs at 100Hz but it's not clear if the hardware runs at this rate.