AP_Compass: devid param desc include ReadOnly

libraries/AP_Compass/AP_Compass.cpp

@@ -260,18 +260,21 @@ const AP_Param::GroupInfo Compass::var_info[] = {
     // @Param: DEV_ID
     // @DisplayName: Compass device id
     // @Description: Compass device id.  Automatically detected, do not set manually
+    // @ReadOnly: True
     // @User: Advanced
     AP_GROUPINFO("DEV_ID",  15, Compass, _state[0].dev_id, 0),
 
     // @Param: DEV_ID2
     // @DisplayName: Compass2 device id
     // @Description: Second compass's device id.  Automatically detected, do not set manually
+    // @ReadOnly: True
     // @User: Advanced
     AP_GROUPINFO("DEV_ID2", 16, Compass, _state[1].dev_id, 0),
 
     // @Param: DEV_ID3
     // @DisplayName: Compass3 device id
     // @Description: Third compass's device id.  Automatically detected, do not set manually
+    // @ReadOnly: True
     // @User: Advanced
     AP_GROUPINFO("DEV_ID3", 17, Compass, _state[2].dev_id, 0),