extmod/modlwip: Register TCP close-timeout callback before closing PCB.

In d5f0c87b this call to tcp_poll() was
added to put a timeout on closing TCP sockets.  But after calling
tcp_close() the PCB may be freed and therefore invalid, so tcp_poll() can
not be used at that point.  As a fix this commit calls tcp_poll() before
closing the TCP PCB.  If the PCB is subsequently closed and freed by
tcp_close() or tcp_abort() then the PCB will not be on any active list and
the callback will not be executed, which is the desired behaviour (the
_lwip_tcp_close_poll() callback only needs to be called if the PCB remains
active for longer than the timeout).