iwlwifi: Fix A band scanning when associated

This patch allows A band to be scanned when driver is associated to AP.
Scan mechanism is that mac80211/cfg80211 requests driver to scan G band
first and then immediately to scan A band. Original code require
driver to wait for 2 seconds after any scan before another scan will be
performed. This caused driver to service G band scan request from
mac80211/cfg80211 but deny the A band scan request.

Signed-off-by: Henry Zhangh <hongx.c.zhang@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>