iwl3945: Use iwlcore scan code

A lot of the scanning related code is duplicated between 3945 and agn. Let's
use the iwlcore one and get rid of the 3945.

Signed-off-by: Samuel Ortiz <samuel.ortiz@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>