cfg80211: allow continuous radar monitoring on offchannel chain

Allow continuous radar detection on the offchannel chain in order
to switch to the monitored channel whenever the underlying driver
reports a radar pattern on the main channel.

Tested-by: Owen Peng <owen.peng@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://lore.kernel.org/r/d46217310a49b14ff0e9c002f0a6e0547d70fd2c.1637071350.git.lorenzo@kernel.org


Signed-off-by: Johannes Berg <johannes.berg@intel.com>