mac80211: clear sdata->radar_required

If ieee80211_vif_use_channel() fails, we have to clear
sdata->radar_required (which we might have just set).

Failing to do it results in stale radar_required field
which prevents starting new scan requests.

Reported-by: Jouni Malinen <j@w1.fi>
Signed-off-by: Eliad Peller <eliad@wizery.com>
[use false instead of 0]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>