iwlagn: fix warning when set WEP key

iwl_clear_station_table will be called every time rxon called.
In this function ucode_key_table is set to 0 even though a static
WEP security is set. This will cause in many warning and might be
an issue if dynamic WEP is set. This patch make sure we keep track
of all existing static WEP when this function is called.

Signed-off-by: Mohamed Abbas <mohamed.abbas@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>