net: ethtool: cable-test: Release RTNL when the PHY isn't found

Use the correct logic to check for the presence of a PHY device, and
jump to a label that correctly releases RTNL in case of an error, as we
are holding RTNL at that point.

Fixes: 3688ff30 ("net: ethtool: cable-test: Target the command to the requested PHY")
Closes: https://lore.kernel.org/netdev/20240827104825.5cbe0602@fedora-3.home/T/#m6bc49cdcc5cfab0d162516b92916b944a01c833fSigned-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Larysa Zaremba <larysa.zaremba@intel.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Link: https://patch.msgid.link/20240827092314.2500284-1-maxime.chevallier@bootlin.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/ethtool/cabletest.c

@@ -342,9 +342,9 @@ int ethnl_act_cable_test_tdr(struct sk_buff *skb, struct genl_info *info)
 	phydev = ethnl_req_get_phydev(&req_info,
 				      tb[ETHTOOL_A_CABLE_TEST_TDR_HEADER],
 				      info->extack);
-	if (!IS_ERR_OR_NULL(phydev)) {
+	if (IS_ERR_OR_NULL(phydev)) {
 		ret = -EOPNOTSUPP;
-		goto out_dev_put;
+		goto out_rtnl;
 	}
 
 	ops = ethtool_phy_ops;