[POWERPC] add check for initialized driver data to pmi driver

This patch adds a check for the private driver data to be initialized.
The bug showed up, as the caller found a pmi device by it's type.
Whereas the pmi driver probes for the type and the name.
Since the name was not as the driver expected, it did not initialize.
A more relaxed probing will be supplied with an extra patch, too.
Signed-off-by: Christian Krafft <krafft@de.ibm.com>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>

arch/powerpc/sysdev/pmi.c

@@ -279,6 +279,9 @@ void pmi_register_handler(struct of_device *device,
 	struct pmi_data *data;
 	data = device->dev.driver_data;
 
+	if (!data)
+		return;
+
 	spin_lock(&data->handler_spinlock);
 	list_add_tail(&handler->node, &data->handler);
 	spin_unlock(&data->handler_spinlock);
@@ -289,10 +292,12 @@ void pmi_unregister_handler(struct of_device *device,
 			    struct pmi_handler *handler)
 {
 	struct pmi_data *data;
+	data = device->dev.driver_data;
 
-	pr_debug("pmi: unregistering handler %p\n", handler);
+	if (!data)
+		return;
 
-	data = device->dev.driver_data;
+	pr_debug("pmi: unregistering handler %p\n", handler);
 
 	spin_lock(&data->handler_spinlock);
 	list_del(&handler->node);