mac80211: avoid looking up tid_tx/tid_rx from timers

There's no need to re-lookup the data structures now that
we actually get them immediately with from_timer(), just
avoid that. The struct has to be valid anyway, otherwise
the timer object itself would no longer be valid, and we
can't have a different version of the struct since only a
single session per TID is permitted.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>