dahdi_cfg: Unlink semaphore on early exit.

If dahdi_cfg is terminated while holding the named semaphore, it is possible to
leave it behind and all subsequenct invocations of dahdi_cfg will block waiting
for it.

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