i40e/i40evf: Fix TSO checksum pseudo-header adjustment

With IPv4 and IPv6 now using the same format for checksums based on the
length of the frame we need to update the i40e and i40evf drivers so that
they correctly account for lengths greater than or equal to 64K.

With this patch the driver should now correctly update checksums for frames
up to 16776960 in length which should be more than large enough for all
possible TSO frames in the near future.

Signed-off-by: Alexander Duyck <aduyck@mirantis.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>