stmmac: intel: Fix kernel crash due to wrong error path

Unfortunately sometimes ->probe() may fail. The commit b9663b7c
("net: stmmac: Enable SERDES power up/down sequence")
messed up with error handling and thus:

[   12.811311] ------------[ cut here ]------------
[   12.811993] kernel BUG at net/core/dev.c:9937!

Fix this by properly crafted error path.

Fixes: b9663b7c ("net: stmmac: Enable SERDES power up/down sequence")
Cc: Voon Weifeng <weifeng.voon@intel.com>
Cc: Ong Boon Leong <boon.leong.ong@intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/stmicro/stmmac/stmmac_main.c

@@ -4991,7 +4991,7 @@ int stmmac_dvr_probe(struct device *device,
 						 priv->plat->bsp_priv);
 
 		if (ret < 0)
-			return ret;
+			goto error_serdes_powerup;
 	}
 
 #ifdef CONFIG_DEBUG_FS
@@ -5000,6 +5000,8 @@ int stmmac_dvr_probe(struct device *device,
 
 	return ret;
 
+error_serdes_powerup:
+	unregister_netdev(ndev);
 error_netdev_register:
 	phylink_destroy(priv->phylink);
 error_phy_setup: