iwlwifi: fix bug to show hidden APs during scan

Indirect scanning ('iwlist scan') should report information about
hidden APs. When an AP is hidden it does not respond to active scanning,
we thus have to use passive scanning to locate these APs.

This fixes http://bughost.org/bugzilla/show_bug.cgi?id=1499

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Tested-by: Bill Moss <bmoss@clemson.edu>
Signed-off-by: John W. Linville <linville@tuxdriver.com>