Copter: set RC_SPEED default to 16kHz for brushed

ArduCopter/system.cpp

@@ -692,6 +692,11 @@ void Copter::allocate_motors(void)
         break;
     }
 
+    // brushed 16kHz defaults to 16kHz pulses
+    if (motors->get_pwm_type() >= AP_Motors::PWM_TYPE_BRUSHED) {
+        g.rc_speed.set_default(16000);
+    }
+    
     if (upgrading_frame_params) {
         // do frame specific upgrade. This is only done the first time we run the new firmware
 #if FRAME_CONFIG == HELI_FRAME