Plane: Use AIRSPEED_STALL for roll stall prevent.

Similarly to how it's done in the TECS stall prevention, used
AIRSPEED_STALL (if available) in the stall prevention calculations.

Author: rubenp02

ArduPlane/Attitude.cpp

@@ -659,8 +659,11 @@ void Plane::update_load_factor(void)
     }
 #endif
 
-    float max_load_factor = sq(smoothed_airspeed / MAX(aparm.airspeed_min, 1));
+    float airspeed_stall = is_positive(aparm.airspeed_stall)
+        ? aparm.airspeed_stall
+        : aparm.airspeed_min;
 
+    float max_load_factor = sq(smoothed_airspeed / MAX(airspeed_stall, 1));
     if (max_load_factor <= 1) {
         // our airspeed is below the minimum airspeed. Limit roll to
         // 25 degrees

ArduPlane/Parameters.cpp

@@ -323,7 +323,7 @@ const AP_Param::Info Plane::var_info[] = {
 
     // @Param: AIRSPEED_STALL
     // @DisplayName: Stall airspeed
-    // @Description: If stall prevention is enabled this speed is used to calculate the minimum airspeed while banking. If this is set to 0 then the stall speed is assumed to be the minimum airspeed speed. Typically set slightly higher then true stall speed. Value is as an indicated (calibrated/apparent) airspeed.
+    // @Description: If stall prevention is enabled this speed is used for its calculations. If set to 0 then AIRSPEED_MIN is used instead. Typically set slightly higher than the true stall speed. Value is as an indicated (calibrated/apparent) airspeed.
     // @Units: m/s
     // @Range: 5 75
     // @User: Standard