net: ethtool: break association of ETH_FLAG_* with NETIF_F_*

This is the only place left where dev->features are directly
exposed to userspace.

I know checkpatch.pl complains about __ethtool_{get,set}_flags(), but
the code is easier to read this way.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Acked-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>