Changes to make Hubsan H107L fly

I tried to use the hackocopter firmware for the Hubsan X4 H107L. Trying to fly it just got completely out of control.

Main issues:
 * Motors on Hubsan spin the other way round. PID yaw output has to be reversed.
 * Accelerometer stuff was not fully implemented. Result hard-coded to zero, wrong orientation, conversion to fixedpoint not correct

The changes provided here fix this and try not to interfere with code for the other models.

Thanks to everybody involved in this project!