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.

Submitted by: Augustin Cavalier <waddlesplash@gmail.com> (Haiku)
Obtained from: DragonFlyBSD (5d6b465e288ac5b52d7115688d4e6516acbbea1c)