LinuxKPI: 802.11: implement ieee80211_is_pspoll()

Implement ieee80211_is_pspoll() and move it up to the set of implemented
ieee80211_is_*() functions.

There are no further changes (git diff just looks weird).

Sponsored by: The FreeBSD Foundation
Approved by: re (cperciva)

(cherry picked from commit ff76cbc81d57ffc4bbc9bc20fd22325b2a8c2028)
(cherry picked from commit 2336248a8ba9763ad38074db40f32e117edd86f5)
(cherry picked from commit 16dc23caf17f8ffa21e365a1f2fe1a72d7fecbe8)