dahdi: Initialize the channels cdev structure.

This is necessary to prevent a crash when opening files by the new
device files, which do not have a valid file_operations structure.
This fixes a problem does not exist in any releases of DAHDI.

Since none of the existing tools or applications open files from the
new location of /dev/dahdi/channels/<span>/<offset>, this wasn't
seen until just recently.

Reported-by: Ted Gerold <ted@twg.org>
Signed-off-by: Shaun Ruffell <sruffell@digium.com>