net: dsa: slave: chip data is optional, don't dereference NULL

The new binding does not make use of dsa_chip_data, a.k.a cd.  When
retrieving the size of the EEPROM attached to a switch, don't assume
there is a cd attached to the switch structure.
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dsa/slave.c

@@ -615,7 +615,7 @@ static int dsa_slave_get_eeprom_len(struct net_device *dev)
 	struct dsa_slave_priv *p = netdev_priv(dev);
 	struct dsa_switch *ds = p->parent;
 
-	if (ds->cd->eeprom_len)
+	if (ds->cd && ds->cd->eeprom_len)
 		return ds->cd->eeprom_len;
 
 	if (ds->drv->get_eeprom_len)