drivers/isdn/mISDN/stack.c: remove invalid reference to list iterator variable

drivers/isdn/mISDN/stack.c: remove invalid reference to list iterator variable

If list_for_each_entry, etc complete a traversal of the list, the iterator
variable ends up pointing to an address at an offset from the list head,
and not a meaningful structure.  Thus this value should not be used after
the end of the iterator.  The dereferences are just deleted from the
debugging statement.

This problem was found using Coccinelle (http://coccinelle.lip6.fr/).

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>