Copter: disable hover throttle learn in standby mode

ArduCopter/Attitude.cpp

@@ -9,7 +9,7 @@
 void Copter::update_throttle_hover()
 {
     // if not armed or landed exit
-    if (!motors->armed() || ap.land_complete) {
+    if (!motors->armed() || ap.land_complete || standby_active) {
         return;
     }
 

ArduCopter/standby.cpp

@@ -9,6 +9,7 @@
 //      crash_check is disabled
 //      thrust_loss_check is disabled
 //      parachute_check is disabled
+//      hover throttle learn is disabled
 //      and landing detection is disabled.
 void Copter::standby_update()
 {