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.

Fixes: 933c504424a2 ("firmware: arm_scmi: add scmi protocol bus to enumerate protocol devices")
Reported-by: Uwe Kleine-König <redacted>
Signed-off-by: Sudeep Holla <redacted>
---
 drivers/firmware/arm_scmi/bus.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/firmware/arm_scmi/bus.c b/drivers/firmware/arm_scmi/bus.c
index dc113ad37ad9..6c7e24935eca 100644
--- a/drivers/firmware/arm_scmi/bus.c
+++ b/drivers/firmware/arm_scmi/bus.c

@@ -134,6 +134,9 @@ int scmi_driver_register(struct scmi_driver *driver, struct module *owner,
 {
 	int retval;
 
+	if (!driver->probe)
+		return -EINVAL;
+
 	retval = scmi_protocol_device_request(driver->id_table);
 	if (retval)
 		return retval;