ACPI: If _TSS exists, do not access FADT.duty_width

Factor out legacy FADT.duty_width code
and run it only in the non _TSS case.

Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
Signed-off-by: Li Shaohua <shaohua.li@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>