mac80211 does not set PM field for normal data frames, so we need to
update that based on the current PS mode when using PS-Poll
(timeout=0) power save mode.

This allows the AP to remain in sync with our PS state. However, there
is still a potential race condition between PS state changes when
multiple TX queues are used and nullfunc and PS-Poll frames use
different queue. That corner case may need to be handled separately by
changing which queue is used either in ath9k or mac80211.
Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>