How disable attachment of sio(4) driver to device?

[M. Warner Losh]

In message: <200510241120.31203.jhb at freebsd.org>
            John Baldwin <jhb at freebsd.org> writes:
: Only if you make sure that sio(4) doesn't try to pass data between probe() and 
: attach() via the softc since the softc is preserved from probe to attach if 
: you return 0 from probe.

sio's probe routine, now that you mention this, I believe leaves
things in a state that sioattach knows abou.

However, it looks like the probe routine will get called twice when
you return < 0, so maybe this is OK.

Warner