i40e: do TSO only if CHECKSUM_PARTIAL is set

Don't bother trying to set up a TSO if the skb->ip_summed is not
set to CHECKSUM_PARTIAL.

Change-ID: I6495b3568e404907a2965b48cf3e2effa7c9ab55
Signed-off-by: Shannon Nelson <shannon.nelson@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>