dpif: geneve: supply dpif function to get ifindex

dpif: geneve: supply dpif function to get ifindex

Geneve tunnels are not given a netdev_class function to determine their
ifindex. This means when ofproto-dpif attempts to add a geneve netdev
it fails in 'netdev_ports_insert' in netdev.c. Failure to add this means
that further operations like offloading a rule that egresses to a geneve
port will be rejected as the egress port cannot be found. This patch
applies the same ifindex function to geneve as is used in vxlan.

Signed-off-by: John Hurley <john.hurley@netronome.com>
Acked-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: Simon Horman <simon.horman@netronome.com>