iwlagn: show current rate scale data in debugfs

Add "rate_scale_data" debugfs file to show current bit rate (HT and Legacy),
plus additional information (rssi, noise, tsf, beacon time stamp).

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