greybus: operation: add completion work queue

Add dedicated bound work queue for operation completions and use the
connection work queues for incoming requests only.

There is no need to keep responses ordered internally or with respect to
requests. Instead allow operations to complete as soon as possible when
a response arrives (or the operation is cancelled).

Note that this also allows synchronous requests to be submitted from
request handlers as responses will no longer be blocked on the same
single-threaded work queue. Similarly, operations can now also be
cancelled from a request handler.

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>