greybus: camera: fix memory leak in capture-request handler

Fix memory leak in capture-request handler by making sure to release the
operation request buffer after sending the request.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>