device: Always use FpFinger instead of a mixture of it and finger numbers

We already use FpFinger for storage operations and prints management,
but internally we keep still using the old finger number, that uses
different values for invalid data.

Let's be consistent, and always use FpFinger everywhere.

src/file_storage.c

@@ -251,7 +251,7 @@ static GSList *scan_dev_storedir(char *devpath,
 			continue;
 		}
 
-		list = g_slist_prepend(list, GINT_TO_POINTER(val));
+		list = g_slist_prepend (list, GUINT_TO_POINTER (val));
 	}
 
 	g_dir_close(dir);