w1 - fix fops in w1_bus_notify

Introduce a check to make sure that fops are only called if they have
been defined by the slave module.

Without this check modules like w1_smem cause a NULL pointer dereference
bug.

Signed-off by: Hans-Frieder Vogt <hfvogt@gmx.net>
Acked-by: Evgeniy Polyakov <zbr@ioremap.net>
Cc: stable <stable@vger.kernel.org> # 3.11+
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/w1/w1.c

@@ -613,6 +613,9 @@ static int w1_bus_notify(struct notifier_block *nb, unsigned long action,
 	sl = dev_to_w1_slave(dev);
 	fops = sl->family->fops;
 
+	if (!fops)
+		return 0;
+
 	switch (action) {
 	case BUS_NOTIFY_ADD_DEVICE:
 		/* if the family driver needs to initialize something... */