One-way dbus properties

[Adriana Kobylak]

Wanted to get thoughts on how attempting to clear one-way dbus 
properties should be handled.

There are some properties like Field Mode that are intended to be 
one-way, meaning that you can set them but not clear them:

- 
https://github.com/openbmc/phosphor-dbus-interfaces/blob/master/xyz/openbmc_project/Control/FieldMode.interface.yaml
       description: >
           Keeps track of whether the BMC is in field mode. Enabling 
field mode
           is intended to be a one-way operation.

If there's a call to clear the property value, currently this is a no-op 
and the REST call returns success but the value is not changed.
Should an error be returned to the user? It's like a read-only property 
but only after it's being set the first time.