greybus: operation: refactor response handling

Send response to incoming requests from the operation request handler
rather than in every protocol request_recv callback.

This simplifies request_recv error handling and allows for further code
reuse.

Note that if we ever get protocols that need to hold off sending
responses we could implement this by letting them return a special
value (after acquiring the necessary operation references) to suppress
the response from being sent by greybus core.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>