iwlwifi: mvm: rs: reinit rs if no tx for a long time

After being idle for a long time (>5sec) the rs statistics
will be stale so we prefer to reset rs and start from legacy
rates again. This gives better results when the attenuation
increased signficantly (e.g. we got further from the AP) and
after a while we start Tx
Note that the first Tx after the idle period will still go out
in the old modulation and rate but this seemed a simpler approach
compared to adding a timer or modifying mac80211 for this.
The negative impact is negligble as we'll recover quickly.

Cc: <stable@vger.kernel.org> [3.14]
Signed-off-by: Eyal Shapira <eyalx.shapira@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>

drivers/net/wireless/iwlwifi/mvm/rs.h

@@ -311,6 +311,7 @@ struct iwl_lq_sta {
 	u32 visited_columns;    /* Bitmask marking which Tx columns were
 				 * explored during a search cycle
 				 */
+	u64 last_tx;
 	bool is_vht;
 	enum ieee80211_band band;