greybus: connection: verify disabled when destroyed

A connection must be in DISABLED state before it gets destroyed.
Warn if this is ever not the case (and do the disconnect) before
proceeding with connection destruction.

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