FieldMode: Add NotAllowed error

The FieldMode property is a one-way property, add support to return a
NotAllowed error when it's requested to be cleared.

Tested: Verify an error is created when a value of 0 was requested via
        curl -b cjar -k -H "X-Auth-Token: $sha" -X PUT -d '{"data":0}' https://$bmc/xyz/openbmc_project/software/attr/FieldModeEnabled

Change-Id: I7121026f3dd63eef5a14e4b932ae7649e3c273e6
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
1 file changed