D 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>