ide-cd: rename cdrom_* functions to ide_cd_*

* cdrom_prepare_request() -> ide_cd_init_rq()
* cdrom_queue_packet_command() -> ide_cd_queue_pc()
* cdrom_lockdoor() -> ide_cd_lockdoor()
* cdrom_read_toc() -> ide_cd_read_toc()
* cdrom_get_toc_entry() -> ide_cd_get_toc_entry()

This is a preparation to move code handling cdrom.c IOCTLs out of ide-cd.c.

Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>