ACPICA: Executer: Fix trivial issues in acpi_get_serial_access_bytes()

This patch fixes trivial issues in acpi_get_serial_access_bytes(), no real
functional bugs. 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>