SITL: change SIM_ADSB default to -1 to default disabled to leave telem port available

libraries/AP_HAL_SITL/SITL_State.cpp

@@ -97,7 +97,10 @@ void SITL_State::_sitl_setup(const char *home_str)
             gimbal = new SITL::Gimbal(_sitl->state);
         }
 
-        adsb = new SITL::ADSB(_sitl->state, home_str);
+        if (_sitl->adsb_plane_count >= 0) {
+            adsb = new SITL::ADSB(_sitl->state, home_str);
+        }
+
         fg_socket.connect("127.0.0.1", 5503);
     }
 

libraries/SITL/SITL.cpp

@@ -76,7 +76,7 @@ const AP_Param::GroupInfo SITL::var_info[] = {
     AP_GROUPINFO("ACC2_RND",      42, SITL,  accel2_noise, 0),
     AP_GROUPINFO("ARSPD_FAIL",    43, SITL,  arspd_fail, 0),
     AP_GROUPINFO("GYR_SCALE",     44, SITL,  gyro_scale, 0),
-    AP_GROUPINFO("ADSB_COUNT",    45, SITL,  adsb_plane_count, 0),
+    AP_GROUPINFO("ADSB_COUNT",    45, SITL,  adsb_plane_count, -1),
     AP_GROUPINFO("ADSB_RADIUS",   46, SITL,  adsb_radius_m, 1000),
     AP_GROUPINFO("ADSB_ALT",      47, SITL,  adsb_altitude_m, 1000),
     AP_GROUPEND