LinuxKPI: 802.11: fill in two more TODOs

Implement ieee80211_is_data_present() and a subset of
ieee80211_is_bufferable_mmpdu() which hopefully is good enough in
the compat code for now.
This is partly in preparation for some TXQ changes coming up soon.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 00614c9c2ddc34f3f9061e87542efb4edbd936a9)