Plane: fix global-pos-int velocity direction to NED

ArduPlane/GCS_Mavlink.cpp

@@ -186,7 +186,7 @@ void Plane::send_location(mavlink_channel_t chan)
         relative_altitude * 1.0e3f,    // millimeters above ground
         vel.x * 100,  // X speed cm/s (+ve North)
         vel.y * 100,  // Y speed cm/s (+ve East)
-        vel.z * -100, // Z speed cm/s (+ve up)
+        vel.z * 100,  // Z speed cm/s (+ve Down)
         ahrs.yaw_sensor);
 }