i40e: add encap csum VF offload flag

Add ENCAP_CSUM offload negotiation flag. Currently VF assumes checksum
offload for encapsulated packets is supported by default. Going forward,
this feature needs to be negotiated with PF before advertising to the
stack. Hence, we need a flag to control it.
This is in regards to prepping up for VF base mode functionality support.

Change-ID: Iaab1f25cc0abda5f2fbe3309092640f0e77d163e
Signed-off-by: Preethi Banala <preethi.banala@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>