firmware: arm_scmi: Return a literal instead of a variable

In this context "return scmi_dev;" and "return NULL;" are equivalent
but it is more readable to return a literal.
Signed-off-by: Dan Carpenter <error27@gmail.com>
Reviewed-by: Cristian Marussi <cristian.marussi@arm.com>
Link: https://lore.kernel.org/r/Y/Yx8pOdf8rNhPVe@kiliSigned-off-by: Sudeep Holla <sudeep.holla@arm.com>

drivers/firmware/arm_scmi/bus.c

@@ -436,7 +436,7 @@ struct scmi_device *scmi_device_create(struct device_node *np,
 	/* Nothing to do. */
 	if (!phead) {
 		mutex_unlock(&scmi_requested_devices_mtx);
-		return scmi_dev;
+		return NULL;
 	}
 
 	/* Walk the list of requested devices for protocol and create them */