openvswitch: Add support for 8021.AD

Change the description of the VLAN tpid field.
Signed-off-by: NThomas F Herbert <thomasfherbert@gmail.com>
Acked-by: NPravin B Shelar <pshelar@ovn.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>