mac80211: split out and decrease probe wait time

The time we wait for a probe response after probing an AP due to
beacon loss is currently the same as the monitoring interval, 2s.
This is far too long, APs should respond to probes within a
fraction of that time. To be able to adjust both values, add a
new constant IEEE80211_PROBE_WAIT, use it for checking the probe
response, and adjust it down to 200ms instead of 2 seconds.

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