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

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: d4f96c2d4f07 ("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.mountain
Signed-off-by: Johannes Berg <johannes.berg@intel.com>