Comment # 2 on bug 964562 from Takashi Iwai

The symptom sounds different from boo#955096.  That bug was about the blockage
of shutdown/reboot while yours is spurious wakeup after shutdown.  This is
mostly due to either ACPI or XHCI wakeup.

Check /proc/acpi/wakeup.  For each "enabled" item, echo the device name, e.g.
if "USB1" shows "enabled", run like
   echo -n USB1 > /proc/acpi/wakeup
Then this item will be disabled.  After disabling all, try to shutdown and
check whether the reboot happens.

Disabling the ACPI wakeup would also disable the wake up after suspend, of
course.  It's just for figuring out the cause.

Another possible spurious wakeup source is USB.  XHCI driver has various
workarounds for such firmware problems.  You can pass quirks option to xhci-hcd
module, e.g. try to pass "xhci_hcd.quirks=0x42000" boot option.