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>