ethtool: fix reference leak in ethnl_set_privflags()

Andrew noticed that some handlers for *_SET commands leak a netdev
reference if required ethtool_ops callbacks do not exist. One of them is
ethnl_set_privflags(), a simple reproducer would be e.g.

  ip link add veth1 type veth peer name veth2
  ethtool --set-priv-flags veth1 foo on
  ip link del veth1

Make sure dev_put() is called when ethtool_ops check fails.

Fixes: f265d799

 ("ethtool: set device private flags with PRIVFLAGS_SET request")
Reported-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>