ide-pmac: bugfix for media-bay support rework

Fix bug introduced by:

commit 2dde7861
Author: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Date:   Fri Apr 18 00:46:23 2008 +0200

    ide: rework PowerMac media-bay support (take 2)
...

[ Yeah, I suck. ]

bay->cd_index shouldn't be changed if IDE devices are not present
or retry operations won't happen.

Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>