vlan: fix bond/team enslave of vlan challenged slave/port

In vlan_uses_dev() check for number of vlan devs rather than existence
of vlan_info. The reason is that vlan id 0 is there without appropriate
vlan dev on it by default which prevented from enslaving vlan challenged
dev.

Reported-by: Jon Stanley <jstanley@rmrf.net>
Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>