i40evf: TCP/IPv6 over Vxlan Tx checksum offload fix

We were checking the outer Protocol flags and deciding the flow for
inner header. This patch fixes that.
This fixes the Tx checksum offload for TCP/IPv6 over vxlan.

Change-ID: I837aaea921d34f71b24c2bc32aaadea5001ddf78
Signed-off-by: Anjali Singhai Jain <anjali.singhai@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>