ACPICA: Resource Manager: Add a pointer cast for a namespace node.

Add a missing cast for a namespace node pointer passed in from
an external interface. Reported Rafael Wysocki.

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>