nfp: enable TSO by default for nfp netdev

We can benefit from TSO when the host CPU is not powerful enough,
so enable it by default now.

Signed-off-by: Yinjun Zhang <yinjun.zhang@corigine.com>
Reviewed-by: Louis Peens <louis.peens@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>