iwl3945: fix scan races

Port following patch to 3945.

"commit 90c4162ff59a3281b6d2f7206740be6217bd6758
Author: Johannes Berg <johannes.berg@intel.com>
Date:   Wed Apr 7 00:21:36 2010 -0700
    iwlwifi: fix scan races"

Signed-off-by: Abhijeet Kolekar <abhijeet.kolekar@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>