net: tcp2: Fixed IS_ENABLED check for NET_TCP_MAX_SEND_WINDOW_SIZE

The #if statement used IS_ENABLED to check if it was defined.
IS_ENABLED will only return true if the value is 1, and false otherwise.
If the NET_TCP_MAX_SEND_WINDOW_SIZE value would be e.g. 8, then the
check would fail.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>