iwlwifi: remove duplicated define

Duplicated define for listen interval (IWL_CONN_MAX_LISTEN_INTERVAL and
IWL_CONN_LISTEN_INTERVAL), remove IWL_CONN_LISTEN_INTERVAL

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Reported-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>