sysfs: bugfix: shorten too long file names

* In old kernels (e.g: 2.6.18) sysfs file names must be shorter
  then KOBJ_NAME_LEN.

* Longer names are truncated and this leads to duplicate names
  which fails device_register()
  Examples: "dahdi!channels!10!10" is truncated to "dahdi!channels!10!1"

* The fix shorten the names and make them fixed length. Example:
            "dahdi!chan!010!010"

* It seems no current userspace tools rely on that name or on
  /dev/dahdi/channels .

Signed-off-by: Tzafrir Cohen <tzafrir.cohen@xorcom.com>
Signed-off-by: Shaun Ruffell <sruffell@digium.com>