i40e: restore NETIF_F_GSO_IPXIP[46] to netdev features

Since commit bacd75cf ("i40e/i40evf: Add capability exchange for
outer checksum", 2017-04-06) the i40e driver has not reported support
for IP-in-IP offloads. This likely occurred due to a bad rebase, as the
commit extracts hw_enc_features into its own variable. As part of this
change, it dropped the NETIF_F_FSO_IPXIP flags from the
netdev->hw_enc_features. This was unfortunately not caught during code
review.

Fix this by adding back the missing feature flags.

For reference, NETIF_F_GSO_IPXIP4 was added in commit 7e13318d
("net: define gso types for IPx over IPv4 and IPv6", 2016-05-20),
replacing NETIF_F_GSO_IPIP and NETIF_F_GSO_SIT.

NETIF_F_GSO_IPXIP6 was added in commit bf2d1df3

 ("intel: Add support
for IPv6 IP-in-IP offload", 2016-05-20).

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>