kunit: Fix a NULL vs IS_ERR() bug

The kunit_device_register() function doesn't return NULL, it returns
error pointers.  Change the KUNIT_ASSERT_NOT_NULL() to check for
ERR_OR_NULL().

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>