Sub: use get_throttle_control_dz in place of throttle->get_control_in

This handles special treatment of throttle rc_channel with type_range in
sub

ArduSub/control_manual.cpp

@@ -30,7 +30,7 @@ void Sub::manual_run()
     motors.set_roll(channel_roll->norm_input_dz()*0.67f);
     motors.set_pitch(channel_pitch->norm_input_dz()*0.67f);
     motors.set_yaw(channel_yaw->norm_input_dz()*0.67f);
-    motors.set_throttle(channel_throttle->norm_input());
+    motors.set_throttle(channel_throttle->norm_input_dz());
     motors.set_forward(channel_forward->norm_input_dz()*0.67f);
     motors.set_lateral(channel_lateral->norm_input_dz()*0.67f);
 }