Copter: ch6 tuning of yaw rate filter

ArduCopter/defines.h

@@ -153,6 +153,7 @@
 #define CH6_RATE_ROLL_FF                53  // body frame roll rate controller FF term
 #define CH6_RATE_YAW_FF                 54  // body frame yaw rate controller FF term
 #define CH6_RATE_MOT_YAW_HEADROOM       55  // motors yaw headroom minimum
+#define CH6_RATE_YAW_FILT               56  // yaw rate input filter
 
 // Acro Trainer types
 #define ACRO_TRAINER_DISABLED   0

ArduCopter/tuning.pde

@@ -200,5 +200,9 @@ static void tuning() {
     case CH6_RATE_MOT_YAW_HEADROOM:
         motors.set_yaw_headroom(tuning_value*1000);
         break;
+
+     case CH6_RATE_YAW_FILT:
+         g.pid_rate_yaw.filt_hz(tuning_value);
+         break;
     }
 }