iwlagn: make iwl_scan_cancel_timeout void

The return value of iwl_scan_cancel_timeout()
isn't used anywhere, so let's just remove it.

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