iwlwifi: correct return code for log_event

When dumping event log in debugfs, iwl_dump_nic_event_log()
should return the correct error code instead of let the calling
function makes it own assumption.

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