mac80211: extend/document powersave API

This modifies hardware flags for powersave to support three different
flags:
 * IEEE80211_HW_SUPPORTS_PS - indicates general PS support
 * IEEE80211_HW_PS_NULLFUNC_STACK - indicates nullfunc sending in software
 * IEEE80211_HW_SUPPORTS_DYNAMIC_PS - indicates dynamic PS on the device

It also adds documentation for all this which explains how to set the
various flags.

Additionally, it fixes a few things:
 * a spot where && was used to test flags
 * enable CONF_PS only when associated again

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/drivers/net/wireless/rt2x00/rt2500usb.c b/drivers/net/wireless/rt2x00/rt2500usb.c
index e992bad..27a6971 100644
--- a/drivers/net/wireless/rt2x00/rt2500usb.c
+++ b/drivers/net/wireless/rt2x00/rt2500usb.c
@@ -1801,7 +1801,9 @@
 	rt2x00dev->hw->flags =
 	    IEEE80211_HW_RX_INCLUDES_FCS |
 	    IEEE80211_HW_HOST_BROADCAST_PS_BUFFERING |
-	    IEEE80211_HW_SIGNAL_DBM;
+	    IEEE80211_HW_SIGNAL_DBM |
+	    IEEE80211_HW_SUPPORTS_PS |
+	    IEEE80211_HW_PS_NULLFUNC_STACK;
 
 	rt2x00dev->hw->extra_tx_headroom = TXD_DESC_SIZE;