Fix gyro calibration for rotated sensors

The calibration routine now uses the raw sensor values instead of the already rotated values.

Details for this issue can be found in the corresponding mailing list topic: https://groups.google.com/d/topic/px4users/GxmPRLZMiAQ/discussion

This commit has been tested and works perfectly on my setup (PX4 FMU+IO stack mounted upside-down).