wifi: mac80211: add debugfs file to display per-phy AQL pending airtime

Now that the global pending airtime is more relevant for airtime fairness,
it makes sense to make it accessible via debugfs for debugging

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>
Link: https://lore.kernel.org/r/20220625212411.36675-6-nbd@nbd.name


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