iwl3945: use iwl_bss_info_changed

3945 can use iwl_bss_info_changed.
A new lib op is created for post_assoicate to distinguish between
3945 and iwlwifi's post_associate operations.

Signed-off-by: Abhijeet Kolekar <abhijeet.kolekar@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>