openvswitch: add NETIF_F_HW_VLAN_STAG_TX to internal dev

The internal device does support 802.1AD offloading since 018c1dda


("openvswitch: 802.1AD Flow handling, actions, vlan parsing, netlink
attributes").

Signed-off-by: Jiri Benc <jbenc@redhat.com>
Acked-by: Pravin B Shelar <pshelar@ovn.org>
Acked-by: Eric Garver <e@erig.me>
Signed-off-by: David S. Miller <davem@davemloft.net>