pata_rdc: parallel scanning needs an extra locking

This is similar change as commit 60c3be38 ("ata_piix: parallel scanning
on PATA needs an extra locking") for ata_piix host driver.

Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>