datapath-windows: Build fix.

Recently we changed the signature of NlAttrParse.
Function OvsGetVport was not updated accordingly.

Fixed the same.

Signed-off-by: Ankur Sharma <ankursharma@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Eitan Eliahu <eliahue@vmware.com>
Acked-by: Nithin Raju <nithin@vmware.com>
Acked-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>