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@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>