Compass: fixed the order of rotations in the compass driver

this should fix the massive heading issues that people have been
reporting. Please test!

libraries/AP_Compass/AP_Compass_HMC5843.cpp

@@ -264,10 +264,10 @@ bool AP_Compass_HMC5843::read()
 
    // rotate to the desired orientation
    Vector3f rot_mag = Vector3f(mag_x,mag_y,mag_z);
-   rot_mag.rotate(_orientation);
    if (product_id == AP_COMPASS_TYPE_HMC5883L) {
 	  rot_mag.rotate(ROTATION_YAW_90);
    }
+   rot_mag.rotate(_orientation);
 
    rot_mag += _offset.get();
    mag_x = rot_mag.x;