greybus: connection: add helper to disable incoming operations

Add helper to disable and flush incoming operations.

This is intended to be used by core to flush any incoming requests
before calling driver disconnect, but could potentially later be
exported for driver use as well.

Note that we currently flush all incoming operation and allow the
request handlers to run, but cancel any responses sent. This may need to
be refined later.

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>