[VLAN]: Don't synchronize addresses while the vlan device is down

While the VLAN device is down, the unicast addresses are not configured
on the underlying device, so we shouldn't attempt to sync them.

Noticed by Dmitry Butskoy <buc@odusz.so-cdu.ru>

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>