enic: add devcmds for vxlan offload

This patch adds devcmds needed for vxlan offload. Implement 3 new devcmd

overlay_offload_ctrl: enable/disable offload
overlay_offload_cfg: update offload udp port number
get_supported_feature_ver: get hw supported offload version. Each
			   version has different bitmap for csum_ok/encap

Signed-off-by: Govindarajulu Varadarajan <gvaradar@cisco.com>
Signed-off-by: David S. Miller <davem@davemloft.net>