iwlwifi: unify tx antenna toggling

TX antenna toggling is requested for management frames in tx and
scanning. I addition toggling in scanning was incorrect;

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