mac80211: send last 3/5 probe requests as unicast

Some buggy APs do not respond to unicast probe requests
or send unicast probe requests very delayed so in the
worst case we should try to send broadcast probe requests,
otherwise we can get disconnected from these APs.

Even if drivers do not have filters to disregard probe
responses from foreign APs mac80211 will only process
probe responses from our associated AP for re-arming
connection monitoring.

We need to do this since the beacon monitor does not
push back the connection monitor by design so even if we
are getting beacons from these type of APs our connection
monitor currently relies heavily on the way the probe
requests are received on the AP. An example of an AP
affected by this is the Nexus One, but this has also been
observed with random APs.

We can probably optimize this later by using null funcs
instead of probe requests.

For more details refer to:

http://code.google.com/p/chromium-os/issues/detail?id=5715



This patch has fixes for stable kernels [2.6.35+].

Cc: stable@kernel.org
Cc: Paul Stewart <pstew@google.com>
Cc: Amod Bodas <amod.bodas@atheros.com>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>