net/ethtool/ioctl: return -EOPNOTSUPP if we have no phy stats

[ Upstream commit 9deb1e9fb88b1120a908676fa33bdf9e2eeaefce ]

It's not very useful to copy back an empty ethtool_stats struct and
return 0 if we didn't actually have any stats. This also allows for
further simplification of this function in the future commits.

Signed-off-by: Daniil Tatianin <d-tatianin@yandex-team.ru>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>
[bananafunction: minor adaption]
Signed-off-by: bananafunction <terathree2@web.de>