iwlwifi: drop struct iwl3945_hw_key

This patch replaces struct iwl3945_hw_key by struct iwl_hw_key.
It's not used directly with any host command therefore removal is trivial

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>