ahci: don't attach if ICH6 is in combined mode

ICH6 R/Ms share PCI ID between piix and ahci modes and we've been
allowing ahci to attach regardless of how BIOS configured it.
However, enabling AHCI mode when the controller is in combined mode
can result in unexpected behavior.  Don't attach if the controller is
in combined mode.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Cc: Bill Nottingham <notting@redhat.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>