ofproto: Wildcard TTL on IP tunnels

There is no need to set the mask on the outer header IP TTL [1]. The only requirement
is that the TTL will be non-zero. Clear the mask in tnl_wc_init().

[1] OVS VXLAN decap rule has full match on TTL for the outer headers?
    http://www.spinics.net/lists/netdev/msg351961.html

Cc: Jesse Gross <jesse@kernel.org>
Cc: Joe Stringer <joe@ovn.org>
Signed-off-by: Haggai Eran <haggaie@mellanox.com>
Signed-off-by: Jesse Gross <jesse@kernel.org>