mac80211_hwsim: Add support for client PS mode

This introduces a debugfs file (ieee80211/phy#/hwsim/ps) that can be
used to force a simulated radio into power save mode. Following values
can be written into this file to change PS mode:
0 = power save disabled (constantly awake)
1 = power save enabled (drop all frames; do not send PS-Poll)
2 = power save enabled (send PS-Poll frames automatically to receive
    buffered unicast frames); not yet fully implemented
3 = manual PS-Poll trigger (send a single PS-Poll frame)

Two different behavior for power save mode processing can be tested:
- move between modes 1 and 0 (i.e., receive all buffered frames at a
  time)
- move to mode 1 and use manual PS-Poll frames (write 3 to the 'ps'
  debugfs file) to fetch power save buffered frames one at a time

Mode 2 (automatic PS-Poll) does not yet parse Beacon frames, but
eventually, it should take a look at TIM IE and send PS-Poll if a
traffic bit is set for our AID.

Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>