greybus: svc: use EREMOTEIO for remote errors

Return -EREMOTEIO consistently for unspecified remote errors (e.g. a
failed DME attribute read due to a remote UniPro error).

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