extmod/modlwip: ioctl POLL: Fix handling of peer closed socket.
Peer-closed socket is both readable and writable: read will return EOF, write - error. Without this poll will hang on such socket. Note that we don't return POLLHUP, based on argumentation in http://www.greenend.org.uk/rjk/tech/poll.html that it should apply to deeper disconnects, for example for networking, that would be link layer disconnect (e.g. WiFi went down).