firmware: arm_ffa: Ensure drivers provide a probe function

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/20210621201652.127611-1-u.kleine-koenig@pengutronix.de
Fixes: e7818584 ("firmware: arm_ffa: Add initial FFA bus support for device enumeration")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>

drivers/firmware/arm_ffa/bus.c

@@ -99,6 +99,9 @@ int ffa_driver_register(struct ffa_driver *driver, struct module *owner,
 {
 	int ret;
 
+	if (!driver->probe)
+		return -EINVAL;
+
 	driver->driver.bus = &ffa_bus_type;
 	driver->driver.name = driver->name;
 	driver->driver.owner = owner;