nfp: move representors' struct net_device_ops to shared code

Apps shouldn't declare their own struct net_device_ops for
representors, this makes sharing code harder.  Add necessary
nfp_app callbacks and move the definition of representors'
struct net_device_ops to common code.

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>