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>

drivers/macintosh/mediabay.c

@@ -556,7 +556,8 @@ static void media_bay_step(int i)
 				printk("mediabay %d, registering IDE...\n", i);
 				pmu_suspend();
 				ide_port_scan(bay->cd_port);
-				bay->cd_index = bay->cd_port->index;
+				if (bay->cd_port->present)
+					bay->cd_index = bay->cd_port->index;
 				pmu_resume();
 			}
 			if (bay->cd_index == -1) {