enic: add vxlan offload on tx path

Define ndo_features_check. Hw supports offload only for ipv4 inner and
ipv4 outer pkt.

Code refactor for setting inner tcp pseudo csum.

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