The stack's 802.11 layer can dump debugging information.
You can enable and disable the dumping by using sysctl settings. If you do:
sysctl -a | grep 80211
with a Wi-Fi driver, you'll see net.link.ieee80211.debug and net.link.ieee80211.vap0.debug. To turn on the debug output, type the following:
sysctl -w net.link.ieee80211.debug = 1 sysctl -w net.link.ieee80211.vap0.debug=0xffffffff
You can then use slog2info to display the debug log.