WPNav: reduce loiter speed used to correct pos error

Contributed by Leonard Hall
This should reduce the aggressiveness of the response when we experience
a GPS glitch

libraries/AC_WPNav/AC_WPNav.cpp

@@ -271,7 +271,7 @@ void AC_WPNav::update_loiter()
     translate_loiter_target_movements(dt);
 
     // run loiter position controller
-    get_loiter_position_to_velocity(dt, _loiter_speed_cms);
+    get_loiter_position_to_velocity(dt, WPNAV_LOITER_SPEED_MAX_TO_CORRECT_ERROR);
 }
 
 /// calculate_loiter_leash_length - calculates the maximum distance in cm that the target position may be from the current location

libraries/AC_WPNav/AC_WPNav.h

@@ -20,6 +20,7 @@
 #define WPNAV_LOITER_SPEED              500.0f      // maximum default loiter speed in cm/s
 #define WPNAV_LOITER_ACCEL_MAX          250.0f      // maximum acceleration in loiter mode
 #define WPNAV_LOITER_ACCEL_MIN           25.0f      // minimum acceleration in loiter mode
+#define WPNAV_LOITER_SPEED_MAX_TO_CORRECT_ERROR 200.0f      // maximum speed used to correct position error (i.e. not including feed forward)
 
 #define MAX_LEAN_ANGLE                  4500        // default maximum lean angle