ethtool: provide netdev features with FEATURES_GET request

Implement FEATURES_GET request to get network device features. These are
traditionally available via ETHTOOL_GFEATURES ioctl request.

v2:
  - style cleanup suggested by Jakub Kicinski

Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>