nfp: allow converting representor's netdev into nfp_port

Based on struct net_device_ops figure out if netdev is a nfp_repr.
Use this knowledge to convert netdev directly to nfp_port.

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>