iwlwifi: suppress unused variable warning when compiling w/o IWLWIFI_DEBUG

This patch adds __maybe_unused attribute to priv variables used in
functions that used it solely for debug printouts

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Cc: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>