Plane: fixed servo jitter due to airspeed estimate from throttle

when we have no other airspeed source and we are armed we get airspeed
from throttle. the default setting of k_throttle in the radio code
caused an oscillation of the value used in the airspeed estimate, each
time a RC frame came in.

ArduPlane/servos.cpp

@@ -386,16 +386,21 @@ void Plane::set_servos_controlled(void)
             // manual pass through of throttle while throttle is suppressed
             SRV_Channels::set_output_scaled(SRV_Channel::k_throttle, get_throttle_input(true));
         }
-    } else if (g.throttle_passthru_stabilize && 
-               (control_mode == STABILIZE || 
-                control_mode == TRAINING ||
-                control_mode == ACRO ||
-                control_mode == FLY_BY_WIRE_A ||
-                control_mode == AUTOTUNE) &&
-               !failsafe.throttle_counter) {
-        // manual pass through of throttle while in FBWA or
-        // STABILIZE mode with THR_PASS_STAB set
-        SRV_Channels::set_output_scaled(SRV_Channel::k_throttle, get_throttle_input(true));
+    } else if (control_mode == STABILIZE ||
+               control_mode == TRAINING ||
+               control_mode == ACRO ||
+               control_mode == FLY_BY_WIRE_A ||
+               control_mode == AUTOTUNE) {
+        // a manual throttle mode
+        if (failsafe.throttle_counter) {
+            SRV_Channels::set_output_scaled(SRV_Channel::k_throttle, 0);
+        } else if (g.throttle_passthru_stabilize) {
+            // manual pass through of throttle while in FBWA or
+            // STABILIZE mode with THR_PASS_STAB set
+            SRV_Channels::set_output_scaled(SRV_Channel::k_throttle, channel_throttle->get_control_in_zero_dz());
+        } else {
+            SRV_Channels::set_output_scaled(SRV_Channel::k_throttle, channel_throttle->get_control_in());
+        }
     } else if ((control_mode == GUIDED || control_mode == AVOID_ADSB) &&
                guided_throttle_passthru) {
         // manual pass through of throttle while in GUIDED