nfp: flower: set ip tunnel ttl from encap action

The TTL for encapsulating headers in IPv4 UDP tunnels is taken from a
route lookup. Modify this to first check if a user has specified a TTL to
be used in the TC action.

Signed-off-by: John Hurley <john.hurley@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>