net: tcp: Use default MSS value if no MSS option is present

In case peer does not send the MSS option, the TCP stack should assume
default peer MSS value of 536, as per RFC 1122:

    If an MSS option is not received at connection setup, TCP
    MUST assume a default send MSS of 536.

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