This adds an API to mac80211 to handle scheduling of TXQs. The interface
between driver and mac80211 for TXQ handling is changed by adding two new
functions: ieee80211_next_txq(), which will return the next TXQ to schedule
in the current round-robin rotation, and ieee80211_return_txq(), which the
driver uses to indicate that it has finished scheduling a TXQ (which will
then be put back in the scheduling rotation if it isn't empty).

The driver must call ieee80211_txq_schedule_start() at the start of each
scheduling session, and ieee80211_txq_schedule_end() at the end. The API
then guarantees that the same TXQ is not returned twice in the same
session (so a driver can loop on ieee80211_next_txq() without worrying
about breaking the loop.

Usage of the new API is optional, so drivers can be ported one at a time.
In this patch, the actual scheduling performed by mac80211 is simple
round-robin, but a subsequent commit adds airtime fairness awareness to the
scheduler.

Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>
[minor kernel-doc fix, propagate sparse locking checks out]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>