iwl3945 : Fix ad-hoc mode for 3945

Patch fixes the ad-hoc mode by
1) Removing redundant clear_stations_table which prevented generation of
beacons.
2) Setting assoc_id to 1. It was never set so preventing tx flow
in iwl3945_tx_skb.

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>