net: stmmac: manage errors returned by of_get_mac_address()

Commit d01f449c ("of_net: add NVMEM support to of_get_mac_address")
added support for reading the MAC address from an nvmem-cell. This
required changing the logic to return an error pointer upon failure.

If stmmac is loaded before the nvmem provider driver then
of_get_mac_address() return an error pointer with -EPROBE_DEFER.

Propagate this error so the stmmac driver will be probed again after the
nvmem provider driver is loaded.
Default to a random generated MAC address in case of any other error,
instead of using the error pointer as MAC address.

Fixes: d01f449c ("of_net: add NVMEM support to of_get_mac_address")
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

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

@@ -370,6 +370,13 @@ stmmac_probe_config_dt(struct platform_device *pdev, const char **mac)
 		return ERR_PTR(-ENOMEM);
 
 	*mac = of_get_mac_address(np);
+	if (IS_ERR(*mac)) {
+		if (PTR_ERR(*mac) == -EPROBE_DEFER)
+			return ERR_CAST(*mac);
+
+		*mac = NULL;
+	}
+
 	plat->interface = of_get_phy_mode(np);
 
 	/* Some wrapper drivers still rely on phy_node. Let's save it while