ide-cd: unify ide_cd_do_request() exit paths

* Move cdrom_end_request() calls from cdrom_start_rw()
  and ide_cd_prepare_rw_request() to ide_cd_do_request().

* Unify ide_cd_do_request() exit paths.

There should be no functional changes caused by this patch.

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