mac80211: disable PS while probing AP

When associated, but probing the AP because we detected
beacon loss, we need to disable powersave to be able to
receive the probe response. Change the code to do that by
checking whether we're trying to probe when determining
the possibility of going into PS, and recalculate the PS
ability at the necessary spots.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>