ide-cd: document capacity hack

Just copy the comment from drivers/scsi/sr.c::sr_done()
(from which the capacity hack has been originated).

Cc: Borislav Petkov <petkovbb@gmail.com>
Acked-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>