iwlwifi: clean key table in iwl_clear_stations_table

Cleans uCode key table bit map iwl_clear_stations_table
since all stations are cleared also the key table must be.

Since the keys are not removed properly on suspend by mac80211
this may result in exhausting key table on resume leading
to memory corruption during removal

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>