greybus: operation: fix memory leak in request_send error path

Make sure to drop the operation reference when sending the request fails
to avoid leaking the operation structures.

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