greybus: Cleanup connection leftovers

This wouldn't happen normally, but in a buggy corner case it is possible
that all the connections aren't removed properly and they are still
present after the interfaces and endo are removed.

Warn in such a case and cleanup connections, so that /sys/bus/greybus/
can be removed while removing greybus modules.

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