greybus: operation: fix use-after-free and infinite loop on unhandled requests

Make sure to return a proper response in case we get a request we do not
recognise.

This fixes an infinite loop and use-after-free bug, where the freed
operations structure would get re-added to the work queue indefinitely.

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