ACPICA: Resource Mgr: Small fix for buffer size calculation

Fixes a one byte error in the output buffer calculation.
Feng Tang - ACPICA BZ 849:
https://www.acpica.org/bugzilla/show_bug.cgi?id=849

Signed-off-by: Feng Tang <feng.tang@intel.com>
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>