iwlagn: fix sparse warning when compiling without debug

C [M]  drivers/net/wireless/iwlwifi/iwl-core.o
drivers/net/wireless/iwlwifi/iwl-core.c:1341: warning:
‘iwl_dump_nic_error_log’ defined but not used

Reported-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>