ACPICA: Tables: Cleanup table checksum verification code.

This patch reduces code redundancy by moving the FACS/S3PT checksum
verification skip logic into acpi_tb_verify_checksum() and other
calls of this function also get benefit from this change.  Lv Zheng.

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