mac80211: airtime: Fix an off by one in ieee80211_calc_rx_airtime()

This code was copied from mt76 and inherited an off by one bug from
there.  The > should be >= so that we don't read one element beyond
the end of the array.

Fixes: db3e1c40 ("mac80211: Import airtime calculation code from mt76")
Reported-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/r/20191126120910.ftr4t7me3by32aiz@kili.mountainSigned-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/airtime.c

@@ -442,7 +442,7 @@ u32 ieee80211_calc_rx_airtime(struct ieee80211_hw *hw,
 			return 0;
 
 		sband = hw->wiphy->bands[status->band];
-		if (!sband || status->rate_idx > sband->n_bitrates)
+		if (!sband || status->rate_idx >= sband->n_bitrates)
 			return 0;
 
 		rate = &sband->bitrates[status->rate_idx];