Copter: use define for pre-arm compass offset check

ArduCopter/motors.pde

@@ -271,7 +271,7 @@ static void pre_arm_checks(bool display_failure)
 
         // check for unreasonable compass offsets
         Vector3f offsets = compass.get_offsets();
-        if(offsets.length() > 500) {
+        if(offsets.length() > COMPASS_OFFSETS_MAX) {
             if (display_failure) {
                 gcs_send_text_P(SEVERITY_HIGH,PSTR("PreArm: Compass offsets too high"));
             }