i40e: fix TCP flag replication for hardware offload

As reported by Eric Dumazet, the i40e driver was allowing the hardware
to replicate the PSH flag on all segments of a TSO operation.

This patch fixes the first/middle/last TCP flags settings which
makes the TSO operations work correctly.

With this change we are now configuring the CWR bit to only be set
in the first packet of a TSO, so this patch also enables TSO_ECN,
in order to advertise to the stack that we do the right thing
on the wire.

Reported-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Kavindya Deegala <kavindya.s.deegala@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>