Tweak to FBW min altitude

Patch from Yury

ArduPlane/ArduPlane.pde

@@ -905,7 +905,7 @@ static void update_current_flight_mode(void)
 				nav_roll = g.channel_roll.norm_input() * g.roll_limit;
 				altitude_error = g.channel_pitch.norm_input() * g.pitch_limit_min;
 
-				if ((current_loc.alt>=home.alt+g.FBWB_min_altitude) || (g.FBWB_min_altitude == -1)) {
+				if ((current_loc.alt>=home.alt+g.FBWB_min_altitude) || (g.FBWB_min_altitude == 0)) {
 	 				altitude_error = g.channel_pitch.norm_input() * g.pitch_limit_min;
 				} else {
 					if (g.channel_pitch.norm_input()<0)

ArduPlane/Parameters.h

@@ -85,7 +85,7 @@ public:
         //
         k_param_flybywire_airspeed_min = 120,
         k_param_flybywire_airspeed_max,
-        k_param_FBWB_min_altitude,  // -1=disabled, minimum value for altitude in cm (default 30m)
+        k_param_FBWB_min_altitude,  // 0=disabled, minimum value for altitude in cm (for first time try 30 meters = 3000 cm)
 
         //
         // 130: Sensor parameters