[8021Q]: transfer dev_id from real device

A net_device struct provides field dev_id. It is used for
unique ipv6 generation in case of shared network cards
(as for the OSA network cards of IBM System z).
If VLAN devices are built on top of such shared network cards,
this dev_id information needs to be transferred to the VLAN device.

Signed-off-by: Ursula Braun <braunu@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>