ahci: Blacklist HP Compaq 6720s that spins off disks during ACPI power off

Blacklist HP Compaq 6720s so that it doesn't play a "spin down,
spin up, spin down" ping-pong with the hard disk during system
power off.

Signed-off-by: Maciej Rutecki <maciej.rutecki@gmail.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>