greybus: core: add defensive connection disable post disconnect

Bundle drivers *must* disable their connections in the disconnect
callback, but add a defensive test and warn about buggy drivers
nonetheless.

Note that bundle drivers would generally release their state containers
in disconnect so a failure stop I/O could potentially lead to
use-after-free bugs in any late operation completion callbacks.

Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>