iwlwifi: improve scan support

This modifies iwlwifi to
 * no longer build its own probe request, but use mac80211's
 * therefore, support arbitrary scan IEs (up to the max len)
 * support multiple scan SSIDs
 * support passive scanning

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>