[connman] loosen the autoconnect failure clamp.

This fixes the condition when on the edge of a low signal, the
autoconnect keeps attempting to connect, Whereas before it would try
once, and then be locked out, until user manually connected.
Doesn't this combine two previously separate states: failure_connect_interval == 0 and failure_connect_interval < 0

failure_connect_interval == -1 was being used as a stop retrying signal. This is no longer the case as if it is -1 it will now be set back to 5 the next time this function is called.

That might not necessarily be a problem, but it does mean that wherever it is set to -1 (@ lines 4017 and 5578) and is no longer necessary. Those will in effect reset the time out back to the quickest (5 seconds) once auto_connect_service() is next invoked.