ata_piix: convert to prepare - activate initialization

ata_piix requires more configuration during initialization than most
other SFF compliant controllers and one-go initialzation with
ata_pci_one() is too rigid.

This patch converts ata_piix to use two step prepare - activate
initialization used by other more advanced controllers.  This
conversion brings the happy side effect of enabling PCI device before
accessing PCI configuration registers.  Other than that, there
shouldn't be any behavior change.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>