if_iwm - Update firmware rs table, instead of indexing the table in tx cmds.

if_iwm - Update firmware rs table, instead of indexing the table in tx cmds.

* Rather than providing a non-zero index into the firmware RS table,
  we should always use index 0 and update the firmware RS table whenever
  our chosen tx rate for data-frames changes.

* Send IWM_LQ_CMD updates when the tx rate gets updated by the net80211
  rate control (which is after we tell the tx status to the net80211
  rate-control in iwm_mvm_rx_tx_cmd_single()).

* Disregard frames transferred with a different tx rate than the currently
  selected rate for the rate-control calculations. This way we avoid
  counting management frames (which are sent at a slow, and fixed rate),
  as well as frames we added to the tx queue just before a new IWM_LQ_CMD
  update took effect.

Inspired-By: Linux iwlwifi