AP_GPS: NOVA: fix undulation

Required for the ellipsoid height to be broadcast correctly over e.g.
MAVLink. Tested on a NovAtel OEM719.

Author: tpwrules

libraries/AP_GPS/AP_GPS_NOVA.cpp

@@ -211,7 +211,7 @@ AP_GPS_NOVA::process_message(void)
         state.location.lat = (int32_t) (bestposu.lat * (double)1e7);
         state.location.lng = (int32_t) (bestposu.lng * (double)1e7);
         state.have_undulation = true;
-        state.undulation = bestposu.undulation;
+        state.undulation = -bestposu.undulation;
         set_alt_amsl_cm(state, bestposu.hgt * 100);
 
         state.num_sats = bestposu.svsused;