AP_AHRS: use compass get_{field,offsets}() functions

Both functions are equivalent, so we're going to simply use
get_{field,offsets}() instead of get_{field,offsets}_milligauss().

libraries/AP_AHRS/AP_AHRS_DCM.cpp

@@ -276,7 +276,7 @@ AP_AHRS_DCM::normalize(void)
 float
 AP_AHRS_DCM::yaw_error_compass(void)
 {
-    const Vector3f &mag = _compass->get_field_milligauss();
+    const Vector3f &mag = _compass->get_field();
     // get the mag vector in the earth frame
     Vector2f rb = _dcm_matrix.mulXY(mag);