nfp: make the representor get stats app-independent

Thanks to the fact that all representors will now have an nfp_port,
we can depend on information there to provide a app-independent
.ndo_get_stats64().

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>