openvswitch: correct encoding of set tunnel action attributes

In a set action tunnel attributes should be encoded in a
nested action.

I noticed this because ovs-dpctl was reporting an error
when dumping flows due to the incorrect encoding of tunnel attributes
in a set action.

Fixes: fc4099f1 ("openvswitch: Fix egress tunnel info.")
Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>