[libata] ata_piix: Consolidate PCS register writing

Prior to this patch, the driver would do this for each port:
	read 8-bit PCS
	write 8-bit PCS
	read 8-bit PCS
	write 8-bit PCS

In the field, flaky behavior has been observed related to this register.
In particular, these overzealous register writes can cause misdetection
problems.

Update to do the following once (not once per port) at boot:
	read 16-bit PCS
	if needs changing,
		write 16-bit PCS

And thereafter, we only perform a 'read 16-bit PCS' per port.

This should eliminate all PCS writes in many cases, and be more friendly
in the cases where we do need to enable ports.

Signed-off-by: Jeff Garzik <jeff@garzik.org>