iwlwifi: fix compile error without debugging support

Encounter compiler error when iwlwifi debugging support is
disabled, fix it.

This compiler error was introduced by the previous WiFi/BT coexist patchset.

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>