ide-disk: move /proc handling to ide-disk_proc.c (take 3)

While at it:
- idedisk_capacity() -> ide_disk_capacity()
- idedisk_proc[] -> ide_disk_proc[]
- idedisk_settings[] -> ide_disk_settings[]

v2/3:
Build fix for CONFIG_IDE_PROC_FS=n from Elias Oltmanns.

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