ide-cd: remove struct atapi_capabilities_page (take 2)

* Remove struct atapi_capabilities_page.

* Add ATAPI_CAPABILITIES_PAGE[_PAD]_SIZE define.

v2:
* The buf[] array access indexes were swapped between 'curspeed'/'maxspeed'.
  (Noticed by Borislav Petkov).

Cc: Borislav Petkov <bbpetkov@yahoo.de>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>