ide: merge ide_disk_special() into do_special() (v2)

While at it:
- change debug printk() level to KERN_DEBUG and use __func__
- update documentation

v2:
- fix DEBUG build (noticed by Sergei)

There should be no functional changes caused by this patch.

Cc: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>