mac80211: monitor the connection

With the recent MLME rework I accidentally removed the connection
monitoring code. In order to add it back, this patch will add new
code to monitor both for beacon loss and for the connection actually
working, with possibly separate triggers.

When no unicast frames have been received from the AP for (currently)
two seconds, we will send the AP a probe request. Also, when we don't
see beacons from the AP for two seconds, we do the same (but those
times need not be the same due to the way the code is now written).

Additionally, clean up the parameters to the ieee80211_set_disassoc()
function that I need here, those are all useless except sdata.

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