ethtool: Don't omit the netlink reply if no features were changed

The legacy ethtool userspace tool shows an error when no features could
be changed. It's useful to have a netlink reply to be able to show this
error when __netdev_update_features wasn't called, for example:

1. ethtool -k eth0
   large-receive-offload: off
2. ethtool -K eth0 rx-fcs on
3. ethtool -K eth0 lro on
   Could not change any device features
   rx-lro: off [requested on]
4. ethtool -K eth0 lro on
   # The output should be the same, but without this patch the kernel
   # doesn't send the reply, and ethtool is unable to detect the error.

This commit makes ethtool-netlink always return a reply when requested,
and it still avoids unnecessary calls to __netdev_update_features if the
wanted features haven't changed.

Fixes: 0980bfcd

 ("ethtool: set netdev features with FEATURES_SET request")
Signed-off-by: Maxim Mikityanskiy <maximmi@mellanox.com>
Reviewed-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>