IB/ipath: return correct PortGUID in NodeInfo

Return the PortGUID of the correct port when responding to a NodeInfo
query.  Returning the SystemImageGUID causes issues when there are
multiple HCAs in a single system.

Signed-off-by: Sean Hefty <sean.hefty@intel.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>