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