smsc9420: handle magic field of ethtool_eeprom

ethtool.h says the driver should set the magic field in get_eeprom and
verify it in set_eeprom.  This patch adds this functionality using an
arbitary driver-specific magic value constant (0x9420).
Signed-off-by: Steve Glendinning <steve.glendinning@smsc.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/smsc9420.c

@@ -413,6 +413,7 @@ static int smsc9420_ethtool_get_eeprom(struct net_device *dev,
 	}
 
 	memcpy(data, &eeprom_data[eeprom->offset], len);
+	eeprom->magic = SMSC9420_EEPROM_MAGIC;
 	eeprom->len = len;
 	return 0;
 }
@@ -423,6 +424,9 @@ static int smsc9420_ethtool_set_eeprom(struct net_device *dev,
 	struct smsc9420_pdata *pd = netdev_priv(dev);
 	int ret;
 
+	if (eeprom->magic != SMSC9420_EEPROM_MAGIC)
+		return -EINVAL;
+
 	smsc9420_eeprom_enable_access(pd);
 	smsc9420_eeprom_send_cmd(pd, E2P_CMD_EPC_CMD_EWEN_);
 	ret = smsc9420_eeprom_write_location(pd, eeprom->offset, *data);

drivers/net/smsc9420.h

@@ -44,6 +44,7 @@
 #define LAN_REGISTER_EXTENT		(0x400)
 
 #define SMSC9420_EEPROM_SIZE		((u32)11)
+#define SMSC9420_EEPROM_MAGIC		(0x9420)
 
 #define PKT_BUF_SZ			(VLAN_ETH_FRAME_LEN + NET_IP_ALIGN + 4)