greybus: operation: use per-connection work queues

Replace the global operation work queue with per-connection work queues.

There is no need to keep operations strictly ordered across connections,
something which only adds unnecessary latency.

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