ath9k: Set PM field in frame control when in PS mode

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>