i40e: Remove unnecessary sw_ring access from xsk Tx

Remove the unnecessary access to the software ring for the AF_XDP
zero-copy driver. This was used to record the length of the packet so
that the driver Tx completion code could sum this up to produce the
total bytes sent. This is now performed during the transmission of the
packet, so no need to record this in the software ring.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/1605525167-14450-3-git-send-email-magnus.karlsson@gmail.com