iwlwifi: add valid tx antenna information in rate_scale_table debugfs

when display rate_scale_table debugfs information, also display valid tx
antenna information, this will help user to select correct antenna when
issue fixed rate debugfs command

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Acked-by: Ben Cahill <ben.m.cahill@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>