[PATCH] ata_piix: implement force_pcs module parameter

This patch implements force_pcs module parameter for ata_piix.  If 1,
PCS is ignored, 2 honored.  As there seem to be quite a few ICHs w/
impaired PCS, this option will be useful for cases where the default
setting doesn't work.

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