iwlwifi: support truly passive scanning

If passive scanning is requested we should not ask the
microcode to do active scanning after detecting traffic
on a channel -- that should only be used when an active
scan is requested but some channels are marked passive.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>