p54: revamp station power save management in access point mode

This patch addresses the problem in:
http://marc.info/?l=linux-wireless&m=122727674810057&w=2



Thanks to Stefan Steuerwald <salsasepp@googlemail.com>
extensive iPod touch tests. We could finally squash some bugs in
p54's master mode / access point implementation.

Let's hope we got everything right this time and all stations
from now on will wake up on TIM and receive their queued frames
and go to sleep again without any hiccups.

Signed-off-by: Christian Lamparter <chunkeey@web.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>