dahdi: Prevent memory corruption on device unload.

The channels now have embedded 'struct devices' that contain a device_private
pointer in which to store the device private data. When a device is
unregistered in the system the device privata data is freed by the device core,
even though the chan_release function doesn't free the memory
associated with the device, So when dev_set_drvdata() is called,
it's writing into freed memory.

We also need zero out the embedded struct device before registration,
since we do not have any guarantee that it points to valid memory (or, if a
channel was unregisterd with sysfs, and then reregistered without being cleared
out).  Otherwise the core could try to use the previously freed private
data again.

Signed-off-by: Shaun Ruffell <sruffell@digium.com>
Signed-off-by: Russ Meyerriecks <rmeyerriecks@digium.com>