kunit: device: Fix a NULL vs IS_ERR() check in init()

The root_device_register() function does not return NULL, it returns
error pointers.  Fix the check to match.

Fixes: d03c720e ("kunit: Add APIs for managing devices")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Rae Moar <rmoar@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

lib/kunit/device.c

@@ -45,8 +45,8 @@ int kunit_bus_init(void)
 	int error;
 
 	kunit_bus_device = root_device_register("kunit");
-	if (!kunit_bus_device)
-		return -ENOMEM;
+	if (IS_ERR(kunit_bus_device))
+		return PTR_ERR(kunit_bus_device);
 
 	error = bus_register(&kunit_bus_type);
 	if (error)