iwl3945: fix internal scan

Port of internal scan to iwl3945 missed introduction
of iwl3945_get_single_channel_for_scan.

Fix the following bug by introducing the iwl3945_get_single_channel_for_scan
http://bugzilla.intellinuxwireless.org/show_bug.cgi?id=2208



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