ACPICA: Properly handle NULL entries in _PRT return packages.

This change enables proper handling of NULL package entries in a _PRT
return value, during construction of the PCI routing table.
Lv Zheng.

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