APM: send airspeed sensor value, not estimate, when enabled

this solves the problem of people reporting that airspeed is not shown
when ARSPD_USE is zero.

ArduPlane/GCS_Mavlink.pde

@@ -340,7 +340,7 @@ static void NOINLINE send_radio_out(mavlink_channel_t chan)
 static void NOINLINE send_vfr_hud(mavlink_channel_t chan)
 {
     float aspeed;
-    if (airspeed.use()) {
+    if (airspeed.enabled()) {
         aspeed = airspeed.get_airspeed();
     } else if (!ahrs.airspeed_estimate(&aspeed)) {
         aspeed = 0;