ethtool: fix error handling in linkstate_prepare_data()

When getting SQI or maximum SQI value fails in linkstate_prepare_data(), we
must not return without calling ethnl_ops_complete(dev) as that could
result in imbalance between ethtool_ops ->begin() and ->complete() calls.

Fixes: 80660219

 ("ethtool: provide UAPI for PHY Signal Quality Index (SQI)")
Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>