Plane: Fix max. load factor formula

The correct approximation for the max. load factor n_max at a specific
airspeed is n_max = (V_current / V_stall)^2. The roll limit was being
calculated correctly because the value was squared in that expression,
but the control flow was based on an incorrect value. This led to
slightly overcautious roll limits.

Adjusted the formula to be correct, and removed the squaring in the roll
limit calculation.

ArduPlane/Attitude.cpp

@@ -657,7 +657,7 @@ void Plane::update_load_factor(void)
     }
 #endif
 
-    float max_load_factor = smoothed_airspeed / MAX(aparm.airspeed_min, 1);
+    float max_load_factor = sq(smoothed_airspeed / MAX(aparm.airspeed_min, 1));
     if (max_load_factor <= 1) {
         // our airspeed is below the minimum airspeed. Limit roll to
         // 25 degrees
@@ -668,13 +668,13 @@ void Plane::update_load_factor(void)
         // load limit. Limit our roll to a bank angle that will keep
         // the load within what the airframe can handle. We always
         // allow at least 25 degrees of roll however, to ensure the
-        // aircraft can be manoeuvred with a bad airspeed estimate. At
+        // aircraft can be manoeuvered with a bad airspeed estimate. At
         // 25 degrees the load factor is 1.1 (10%)
-        int32_t roll_limit = degrees(acosf(sq(1.0f / max_load_factor)))*100;
+        int32_t roll_limit = degrees(acosf(1.0f / max_load_factor))*100;
         if (roll_limit < 2500) {
             roll_limit = 2500;
         }
         nav_roll_cd = constrain_int32(nav_roll_cd, -roll_limit, roll_limit);
         roll_limit_cd = MIN(roll_limit_cd, roll_limit);
-    }    
+    }
 }