nfp: don't enable TSO on the device when disabled

We advertise TSO to the stack but leave it disabled by default.
Make sure it's not only disabled in the netdev features but
also on the device itself.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>