iwlwifi: reduce debug ifdefs using the optimiser

Instead of using #ifdef CONFIG_IWLWIFI_DEBUG, remove the
iwlwifi_mod_params.debug_level variable completely and
make iwl_have_debug_level() always return false in the
non-debug case. This way, the optimiser will elide all
code for it automatically without having to add #ifdefs.

Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>