Disable probe requests during scans in iwx(4) again.

Disable probe requests during scans in iwx(4) again.

While this is working well for many, some people see device timeouts
when using the device unless we disable probe requests during scans.

The issue was a lot more visible on iwx(4) with earlier firmware.
In fact, iwx(4) did ship with probe requests disabled for most of its
existence. I re-enabled them along with a firmware upgrade since I no
longer saw the problem. However, the issue prevails for other people.

I still have no idea what is causing this. I have already spent enough
time trying to track down a proper fix. Unless we receive help from
someone who knows about firmware internals the best we can do is trial
and error. The problem also existed on iwm(4) 9k devices which we now
run with probe requests disabled, too.

The only upside of probe requests is that scans can complete faster, with
the downside of a potential privacy leak (the previously selected SSID is
exposed). So, overall, we do not lose much here.

Patch tested for a week by Laurence Tratt who is no longer seeing device
timeouts which were relatively frequent before.