Copter: Reverse circle radius stick input

Change the circle mode radius control so that pitch stuck up (forward) reduces the radius. Pulling the pitch stick back (reverse), increases the radius.

ArduCopter/mode_circle.cpp

@@ -47,7 +47,7 @@ void ModeCircle::run()
         const float radius_current = copter.circle_nav->get_radius();           // circle controller's radius target, which begins as the circle_radius parameter
         const float pitch_stick = channel_pitch->norm_input_dz();               // pitch stick normalized -1 to 1
         const float nav_speed = copter.wp_nav->get_default_speed_xy();          // copter WP_NAV parameter speed
-        const float radius_pilot_change = (pitch_stick * nav_speed) * G_Dt;     // rate of change
+        const float radius_pilot_change = (-pitch_stick * nav_speed) * G_Dt;    // rate of change (inverted so pushing stick up reduces radius, as in moving forward)
         const float radius_new = MAX(radius_current + radius_pilot_change,0);   // new radius target
 
         if (!is_equal(radius_current, radius_new)) {