[libata] pata_atiixp: add locking for parallel scanning

This is similar change as commit 60c3be3 for ata_piix host driver
and while pata_atiixp doesn't enable parallel scan yet the race
could probably also be triggered by requesting re-scanning of both
ports at the same time using SCSI sysfs interface.

[Ported to current tree without other patch dependancies by Alan Cox]

Original is
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
This one is
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>