[POWERPC] Build fix for drivers/macintosh/mediabay.c

This fixes the following build error with CONFIG_BLK_DEV_IDE_PMAC=n:

<--  snip  -->

...
  CC      drivers/macintosh/mediabay.o
/home/bunk/linux/kernel-2.6/git/linux-2.6/drivers/macintosh/mediabay.c: In function 'check_media_bay':
/home/bunk/linux/kernel-2.6/git/linux-2.6/drivers/macintosh/mediabay.c:428: error: 'struct media_bay_info' has no member named 'cd_index'
make[3]: *** [drivers/macintosh/mediabay.o] Error 1

<--  snip  -->

Reported-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>