enic: Check inner ip proto for pseudo header csum

To compute pseudo IP header csum, we need to check the inner header for
encap pkt, not outer IP header.

Also add pseudo csum for IPv6 inner pkt.

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