powerpc/therm_adt746x: Don't access non-existing register

The ADT746x don't have any register at sub-address 0, so better use an
existing register for the initial test read.
Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Colin Leroy <colin@colino.net>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

drivers/macintosh/therm_adt746x.c

@@ -387,7 +387,7 @@ static int probe_thermostat(struct i2c_client *client,
 	i2c_set_clientdata(client, th);
 	th->clt = client;
 
-	rc = read_reg(th, 0);
+	rc = read_reg(th, CONFIG_REG);
 	if (rc < 0) {
 		dev_err(&client->dev, "Thermostat failed to read config!\n");
 		kfree(th);