Invalid cast results in HTTP 500 error returned by the Neutron REST APIs.

I think this is a copy+paste error where a INeutronSubnetAware is incorrectly casted to INeutronPortAware.

Change-Id: I5009b790edbc60e5b143a520d306d984e14fea37
Signed-off-by: Madhu Venugopal <mavenugo@gmail.com>