iwl3945: replace association and beacon hooks with bss_info_changed cb

Let mac80211 and bss_info_changed callback handle the association and
beacon changes.

Signed-off-by: Abhijeet Kolekar <abhijeet.kolekar@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>