openvswitch: Add a missing break statement.

Add a break statement to prevent fall-through from
OVS_KEY_ATTR_ETHERNET to OVS_KEY_ATTR_TUNNEL.  Without the break
actions setting ethernet addresses fail to validate with log messages
complaining about invalid tunnel attributes.

Fixes: 0a6410fb

 ("openvswitch: netlink: support L3 packets")
Signed-off-by: Jarno Rajahalme <jarno@ovn.org>
Acked-by: Pravin B Shelar <pshelar@ovn.org>
Acked-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>