This patch eliminates hardcoded return value of register_cdrom().

It also changes the return value to -EINVAL.
It is more appropriate than -2 (-ENOENT) because it is only
happen invalid usage of register_cdrom() by broken cdrom driver.
Signed-off-by: NAkinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: NJens Axboe <jens.axboe@oracle.com>