firmware: arm_scmi: Ensure drivers provide a probe function

commit 5e469da upstream.

The bus probe callback calls the driver callback without further
checking. Better be safe than sorry and refuse registration of a driver
without a probe function to prevent a NULL pointer exception.

Link: https://lore.kernel.org/r/20210624095059.4010157-2-sudeep.holla@arm.com
Fixes: 933c504 ("firmware: arm_scmi: add scmi protocol bus to enumerate protocol devices")
Reported-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Tested-by: Cristian Marussi <cristian.marussi@arm.com>
Reviewed-by: Cristian Marussi <cristian.marussi@arm.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/firmware/arm_scmi/bus.c

@@ -113,6 +113,9 @@ int scmi_driver_register(struct scmi_driver *driver, struct module *owner,
 {
 	int retval;
 
+	if (!driver->probe)
+		return -EINVAL;
+
 	driver->driver.bus = &scmi_bus_type;
 	driver->driver.name = driver->name;
 	driver->driver.owner = owner;