Comment 1 for bug 1524643

so looking on the code only administrator can update this filed see https://github.com/openstack/neutron/blob/master/etc/policy.json#L83
and nova update the binding:profile see
https://github.com/openstack/nova/blob/master/nova/network/neutronv2/api.py#L763-L779
I also tested it myself via the rest api and the update is working see
http://pastebin.com/TSKEK0zh

so I think you are using the neutron client and it is not implemented see
https://github.com/openstack/python-neutronclient/blob/13b95c83edab2d9966e7eee49bb2eefabb8c3c62/neutronclient/neutron/v2_0/port.py#L310-L313
It is implemented only for create https://github.com/openstack/python-neutronclient/blob/13b95c83edab2d9966e7eee49bb2eefabb8c3c62/neutronclient/neutron/v2_0/port.py#L249-L251

why do you need to update this filed? basically it is intend for nova to pass the pci information.