bonding: allow to add vlans on top of empty bond

This limitation maybe had some reason in the past, but now there is not
one -> removing this.

Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Acked-by: Veaceslav Falico <vfalico@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>