Impact: Reverts sequence of reboot fallbacks

Checkin 14d7ca5c changed the default
reboot method to "pci", a.k.a. port CF9.  Unfortunately this has been
shown to cause lockups on at least two systems for which REBOOT_KBD
worked, both Thinkpads with Intel chipsets.  Checkin
3889d0ce reverted the default, but did
not revert the fallback chain.  This checkin reverts the fallback
chain; port CF9 is now only done by explicit "reboot=pci" or a future
potential DMI key.
Signed-off-by: H. Peter Anvin <hpa@zytor.com>