gdrom: 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 <[e-mail]>
Acked-by: Adrian McMenamin <[e-mail]>
Signed-off-by: Bartlomiej Zolnierkiewicz <[e-mail]>

drivers/cdrom/gdrom.c

···
 	return err;
 }
 
+static int gdrom_audio_ioctl(struct cdrom_device_info *cdi, unsigned int cmd,
+			     void *arg)
+{
+	return -EINVAL;
+}
+
 static struct cdrom_device_ops gdrom_ops = {
 	.open			= gdrom_open,
 	.release		= gdrom_release,
···
 	.media_changed		= gdrom_mediachanged,
 	.get_last_session	= gdrom_get_last_session,
 	.reset			= gdrom_hardreset,
+	.audio_ioctl		= gdrom_audio_ioctl,
 	.capability		= CDC_MULTI_SESSION | CDC_MEDIA_CHANGED |
 				  CDC_RESET | CDC_DRIVE_STATUS | CDC_CD_R,
 	.n_minors		= 1,