viocd: add dummy audio ioctl handler

Make sure audio_ioctl is always defined even if being a dummy function
since the cdrom_ioctl interface assumes its existence and we don't
want to BUG on null ptr on some ioctls like, e.g. CDROMREADTOCENTRY,
CDROMREADTOCHDR etc. when we fix CDC_PLAY_AUDIO checking in cdrom.c.

Signed-off-by: Borislav Petkov <petkovbb@gmail.com>
Acked-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>