net: tcp: Set errno properly if connecting to non listening port

If we try to connect to a port which no socket is listening to,
we will get a packet with "ACK | RST" flags set. In this case
the errno should be ECONNREFUSED instead of ETIMEDOUT like we
used to return earlier.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>