cdrom: support devices that have check_events but not media_changed

Commit 93aae17a ("sr: implement
sr_check_events()") replaced the media_changed op with the
check_events op in drivers/scsi/sr.c

All users that check for the CDC_MEDIA_CHANGED capbility try both
the check_events op and the media_changed op, but register_cdrom()
was requiring media_changed.

This patch fixes the capability checking.

The cdrom_select_disc ioctl is also using the two operations, so
they should be required for CDC_SELECT_DISC too.
Signed-off-by: Simon Arlott <simon@fire.lp0.eu>
Cc: Tejun Heo <tj@kernel.org>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Tested-by: Chris Clayton <chris2553@googlemail.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

drivers/cdrom/cdrom.c

@@ -409,7 +409,8 @@ int register_cdrom(struct cdrom_device_info *cdi)
 	}
 
 	ENSURE(drive_status, CDC_DRIVE_STATUS );
-	ENSURE(media_changed, CDC_MEDIA_CHANGED);
+	if (cdo->check_events == NULL && cdo->media_changed == NULL)
+		*change_capability = ~(CDC_MEDIA_CHANGED | CDC_SELECT_DISC);
 	ENSURE(tray_move, CDC_CLOSE_TRAY | CDC_OPEN_TRAY);
 	ENSURE(lock_door, CDC_LOCK);
 	ENSURE(select_speed, CDC_SELECT_SPEED);