MFC: r208349

Change ad_firmware_geom_adjust() to operate on a struct disk * only and
hook it up to ada(4) also. While at it, rename *ad_firmware_geom_adjust()
to *ata_disk_firmware_geom_adjust() etc now that these are no longer
limited to ad(4). This also allows to maintain a compatibility shim for
pc98_ad_firmware_geom_adjust() for the tier-1 PC98 in stable/8.

Reviewed by: mav (r208349)