net: tcp: Fix deadlock with tcp_conn_close()

While improving thread safety of the TCP stack I've introduced a
possible deadlock scenario, when calling tcp_conn_close() in tcp_in().
This function shall not be called with connection mutex locked, as it
calls registered recv callback internally, which could lead to deadlock
between TCP/socket mutexes.

This commit moves the tcp_conn_close() back where it was originally
called. I've verified that the thread safety is still solid with the
test apps used originally.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>