mac80211: enable IEEE80211_CONF_PS only when associated

Also disable power save when disassociated. It makes no sense to have
power save enabled while disassociated.

iwlwifi seems to have this check in the driver, but it's better to do this
in mac80211 instead.

Signed-off-by: Kalle Valo <kalle.valo@nokia.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>