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).