Plane: Allow AP_Soaring to specify loiter radius in THERMAL mode.

ArduPlane/mode_thermal.cpp

@@ -114,8 +114,10 @@ void ModeThermal::update_soaring()
 
 void ModeThermal::navigate()
 {
-    // Zero indicates to use WP_LOITER_RAD
+    // Soaring library calculates radius from SOAR_THML_BANK.
-    plane.update_loiter(0);
+    const float radius = plane.g2.soaring_controller.get_thermalling_radius();
+
+    plane.update_loiter(radius);
 }
 
 bool ModeThermal::exit_heading_aligned() const