nfp: provide ethtool_drvinfo on representors

Extend representors' ethtool ops to show basic info like firmware
version, driver version, and driver name.

While at it don't set drvinfo.n_stats and drvinfo.regdump_len,
core will invoke appropriate handlers to get those.

A helper is added to turn a netdev into nfp_app for convenience.

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>