greybus: core: disable bundle connections on hot-unplug

Disable bundle connections in core before calling driver disconnect in
case the interface is already gone.

This avoids unnecessary timeouts on hot-unplug when a driver does I/O in
its disconnect callback.

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>