cxgb4: enable inner header checksum calculation

set cntrl bits to indicate whether inner header checksum
needs to be calculated whenever the packet is an encapsulated
packet and enable supported encap features.

Fixes: d0a1299c ("cxgb4: add support for vxlan segmentation offload")
Signed-off-by: Ganesh Goudar <ganeshgr@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>