net/enic: enable GENEVE with options offload

Latest firmware supports Geneve with options offload. Current overlay
offload only supports Geneve with no options. This new feature is
disabled by default, as it is intrusive and cannot be used with flow
API. Add a new devarg 'geneve-opt' so the user can explicitly enable
it.

Signed-off-by: Hyong Youb Kim <hyonkim@cisco.com>
Reviewed-by: John Daley <johndale@cisco.com>