ata_piix: implement IOCFG bit18 quirk

Some notebooks need bit18 of IOCFG to be cleared for the drive bay to
work even though the bit is NOOP according to the datasheet.  This
patch implement IOCFG bit18 quirk and apply it to Clevo M570U.

  http://bugzilla.kernel.org/show_bug.cgi?id=8051

Signed-off-by: Tejun Heo <htejun@gmail.com>
Cc: D. Angelis <dangelis@beta-cae.gr>
Signed-off-by: Jeff Garzik <jeff@garzik.org>