Fix bug that prevented throttle angle correction working correctly when

using different values in different.

The cause was the IMU init code which triggered calculation was never
called after switching profiles - it couldn't be called twice because it
also initialised the compass.

The solution was to decouple compass initialisation from IMU
initialisation and extract the code to recalculate throttle angle scale
to a new method.

src/main/flight/imu.h

@@ -23,3 +23,5 @@ extern int16_t throttleAngleCorrection;
 
 int getEstimatedAltitude(void);
 void computeIMU(void);
+void calculateThrottleAngleScale(uint16_t throttle_correction_angle);
+int16_t calculateThrottleAngleCorrection(uint8_t throttle_correction_value);