iwlwifi: report the rate index as an MCS rate number

If transmit in HT rate, report the rate index as an MCS rate number
instead of an index. so "iw" can display correct BitRate

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>