ixgbe: turn NETIF_F_HW_L2FW_DOFFLOAD off by default

NETIF_F_HW_L2FW_DOFFLOAD allows upper layer net devices such
as macvlan to use queues in the hardware to directly submit and
receive skbs.

This creates a subtle change in the datapath though. One change
being the skb may no longer use the root devices qdisc.

Because users may not expect this we can't enable the feature
by default unless the hardware can offload all the software
functionality above it. So for now disable it by default and
let users opt in.

Signed-off-by: John Fastabend <john.r.fastabend@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>