mac80211: rework locking for txq scheduling / airtime fairness

Holding the lock around the entire duration of tx scheduling can create
some nasty lock contention, especially when processing airtime information
from the tx status or the rx path.
Improve locking by only holding the active_txq_lock for lookups / scheduling
list modifications.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>