AC_Fence: remove timeout for waiting on fence semaphore, make nonblocking

Also move where we indicate we've attempted a load.

libraries/AC_Fence/AC_PolyFence_loader.cpp

@@ -617,13 +617,13 @@ bool AC_PolyFence_loader::load_from_eeprom()
         return false;
     }
 
-    _load_attempted = true;
-
     // find indexes of each fence:
-    if (!get_loaded_fence_semaphore().take(1)) {
+    if (!get_loaded_fence_semaphore().take_nonblocking()) {
         return false;
     }
 
+    _load_attempted = true;
+
     unload();
 
     if (_eeprom_item_count == 0) {