mac80211: speed up AP probing using nullfunc frames

If the nullfunc frame used to probe the AP was not acked, there is no point
in waiting for the probe timeout, so advance to the next try (or disconnect)
immediately.
If we do reach the probe timeout without having received a tx status, the
connection is probably really bad and worth disconnecting.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>