mac80211: introduce set_radar_offchan callback

Similar to cfg80211, introduce set_radar_offchan callback in mac80211_ops
in order to configure a dedicated offchannel chain available on some hw
(e.g. mt7915) to perform offchannel CAC detection and avoid tx/rx downtime.

Tested-by: Evelyn Tsai <evelyn.tsai@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://lore.kernel.org/r/201110606d4f3a7dfdf31440e351f2e2c375d4f0.1634979655.git.lorenzo@kernel.org


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