ACPICA: Update names for walk_namespace callbacks to clarify usage.

Use of "preorder" and "postorder" was incorrect. The callbacks are
simply invoked during tree ascent and descent during the
depth-first walk.

Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>