greybus: svc: Use -EIO instead of -EINVAL for unipro errors

-EIO fits better here, rather than -EINVAL as the arguments to the
routine itself are valid, just that we failed while doing unipro
transfers.

Suggested-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>