net: tcp: fix NET_TCP_CLOSED state propagatation

In net_tcp_release() when a TCP connection is being closed, we
should call net_tcp_change_state instead of net_tcp_set_state.

net_tcp_change_state() will call into net_tcp_set_state() but
also contains logic specific to NET_TCP_CLOSED which unregisters
the context's conn_handler and sends an accept callback (if
present) with -ENETRESET error for user code to handle.

This fixes an EISCONN error returned by net_context_bind() when
a TCP-based net_context was reused.  Due to the conn_handler
not being cleaned up in the TCP code.

Change-Id: I8439a028a1c7ae5fd2a50d11caa9947a0ac6c7d4
Signed-off-by: Michael Scott <michael.scott@linaro.org>