Documentation updates for Documentation/cdrom/cdrom-standard.tex:

cdrom_device_ops:
- add check_events() and generic_packet()

cdrom_device_info:
- add one 'const' modifier
- correct some field descriptions
- add some missing fields
- drop 'kdev_t dev;' field

Also drop <n_discs> sentence from documentation because it is not
referenced anywhere in the kernel header or C files.
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>