net: tcp: Do not send FIN when closing listening sockets

A net context in LISTENING mode waits for incoming connections, once
a new connection is established a new net context is spawned which
is responsible for handling the new connection.
Therefore when closing a LISTENING context it is not useful to send FIN
as it is never connected. Actually closing the connection would be done
by calling close on the spawned net context which is returned by the
accept call.

Signed-off-by: Léonard Bise <leonard.bise@gmail.com>