ata_piix: don't attach to ICH6M in ahci mode

ata_piix when attached to ICH6M in AHCI mode doesn't provide any
benefit over using ahci and has detection problems.  Don't let
ata_piix claim ICH6M if it's in AHCI mode.

This change makes ICH6R the only one which ata_piix can attach to even
when it's in ahci mode which is necessary as some devices don't work
properly under ahci mode.  Drop PIIX_FLAG_AHCI and match the
controller directly so that piix_disable_ahci() is called only for it.

This change makes PIIX_SCC no longer used and it gets dropped too.

This fixes bz 9491.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Cc: Robert M. Albrecht <romal@gmx.de>
Signed-off-by: Jeff Garzik <jeff@garzik.org>