MINOR: connection: remove the double test on xprt_done_cb()

MINOR: connection: remove the double test on xprt_done_cb()

The conn_fd_handler used to have one possible call to this function to
notify about end of handshakes, and another one to notify about connection
setup or error. But given that we're now only performing wakeup calls
after connection validation, we don't need to keep two places to run
this test since the conditions do not change in between.

This patch merges the two tests into a single one and moves the
CO_FL_CONNECTED test appropriately as well so that it's called even
on the error path if needed.