isdn divas: fix proc creation

1. creating proc entry and not saving pointer to PDE and checking it
   is not going to work.
2. if proc entry wasn't created, no reason to remove it on error path.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>