ide-cd: remove unused and write-only struct ide_cd_config_flags fields

unused fields:
* ->writing
* ->reserved

write-only fields:
* ->playmsf_as_bcd
* ->subchan_as_bcd
* ->test_write
* ->supp_disc_present

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