MEDIUM: stream-int: update the endp polling status only at the end of si_cs_recv()

MEDIUM: stream-int: update the endp polling status only at the end of si_cs_recv()

Instead of first indicating that there's more data to read from the
conn_stream then re-adjusting this info along the function, we now
instead set the status according to the subscription status at the
end. It's easier, more accurate, and less sensitive to intermediary
changes.

This will soon allow to remove all the si_cant_put() calls that were
placed in the middle to force a subsequent callback and prevent the
function from subscribing to the mux layer.