AC_Sprayer: move the check for srv_channel assignment from update to run method

It should be enough to do this check only once in run method instead of doing it in update method

libraries/AC_Sprayer/AC_Sprayer.cpp

@@ -93,6 +93,11 @@ void AC_Sprayer::run(const bool activate)
         return;
     }
 
+    // exit immediately if the pump function has not been set-up for any servos
+    if (!SRV_Channels::function_assigned(SRV_Channel::k_sprayer_pump)) {
+        return;
+    }
+
     // set flag indicate whether spraying is permitted:
     // do not allow running to be set to true if we are currently not enabled
     _flags.running = _enabled && activate;
@@ -120,11 +125,6 @@ void AC_Sprayer::update()
         return;
     }
 
-    // exit immediately if the pump function has not been set-up for any servo
-    if (!SRV_Channels::function_assigned(SRV_Channel::k_sprayer_pump)) {
-        return;
-    }
-
     // get horizontal velocity
     Vector3f velocity;
     if (!AP::ahrs().get_velocity_NED(velocity)) {