dock: fix dereference after kfree()

dock_remove() calls kfree() on dock_station so we should use
list_for_each_entry_safe() to avoid dereferencing freed memory.

Found by smatch (http://repo.or.cz/w/smatch.git/).  Compile tested.
Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Len Brown <len.brown@intel.com>

drivers/acpi/dock.c

@@ -1146,9 +1146,10 @@ static int __init dock_init(void)
 static void __exit dock_exit(void)
 {
 	struct dock_station *dock_station;
+	struct dock_station *tmp;
 
 	unregister_acpi_bus_notifier(&dock_acpi_notifier);
-	list_for_each_entry(dock_station, &dock_stations, sibiling)
+	list_for_each_entry_safe(dock_station, tmp, &dock_stations, sibiling)
 		dock_remove(dock_station);
 }