net models: simplify condition

Bond.objectivize() is always called with a non-None bonding name. No
need to complicate an internal condition to validate this.

Change-Id: Ie79511ddc2b9890e832175b046c1f32f609580ec
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28496
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>