pata_artop: unify ->prereset methods

* Unify ->prereset methods for ATP850 and ATP86x[R] chipsets.

* Fix ->prereset documentation while at it.

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