mISDN: Use list_count_nodes()

count_list_member() really looks the same as list_count_nodes(), so use the
latter instead of hand writing it.

The first one return an int and the other a size_t, but that should be
fine. It is really unlikely that we get so many parties in a conference.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>