iwlwifi: Display decoded rate/mcs information

This patch adding MCS information in rate_scale_table, it help for
debugging rate scaling algorithm, easy to understand what is the current
rate scale table and matching modulation, plus the last mcs used for tx.

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>