greybus: svc: pwrmon: validate svc protocol op status when getting rail names

AP should check for Greybus SVC Protocol Operation Status to determine if the
operation was successfully completed by the SVC

Testing Done:
- Successfully getting the rail names in the pwrmon_dummy sandbox branch

Signed-off-by: David Lin <dtwlin@google.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>