mac80211: Always show airtime debugfs file when TXQs are enabled

mac80211: Always show airtime debugfs file when TXQs are enabled

The AQL statistics are displayed in the 'airtime' station debugfs file, but
that file was only shown if a driver has enabled the airtime fairness
feature flag. Since AQL can be enabled without airtime fairness, let's
expose the airtime file whenever TXQs are enabled, so the AQL data can be
read.

Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/r/20191210143417.142964-1-toke@redhat.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>