iwlwifi: don't print module loading error if not modular

If the opmode modules aren't modular, there's no point in
printing an error message that request_module() failed.
This will happen because the probe runs during iwlwifi's
init and the opmode is only added during its init.

Reported-by: Jörg Otte <jrg.otte@gmail.com>
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>