nfp: validate the return code from dev_queue_xmit()

dev_queue_xmit() may return error codes as well as netdev_tx_t,
and it always consumes the skb.  Make sure we always return a
correct netdev_tx_t value.

Fixes: eadfa4c3

 ("nfp: add stats and xmit helpers for representors")
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: John Hurley <john.hurley@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>