wifi: iwlwifi: mei: return error from register when not built

When MEI isn't built, it seems like successfully registering
would be wrong. Change this to an error so that in the rest
of the driver, mei_registered won't be true.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20231004123422.d410a97cddfb.I7891544938d5edd5e6e7d2d99540b3637f2f1b1b@changeidSigned-off-by: Johannes Berg <johannes.berg@intel.com>

drivers/net/wireless/intel/iwlwifi/mei/iwl-mei.h

 /* SPDX-License-Identifier: GPL-2.0-only */
 /*
- * Copyright (C) 2021 - 2022 Intel Corporation
+ * Copyright (C) 2021-2023 Intel Corporation
  */
 
 #ifndef __iwl_mei_h__
@@ -493,7 +493,7 @@ static inline void iwl_mei_set_power_limit(__le16 *power_limit)
 
 static inline int iwl_mei_register(void *priv,
 				   const struct iwl_mei_ops *ops)
-{ return 0; }
+{ return -EOPNOTSUPP; }
 
 static inline void iwl_mei_start_unregister(void)
 {}