mac80211: mesh: mark channel as unusable if a regulatory MESH CSA is received

In the Mesh Channel Switch Parameters (8.4.2.105) the reason is specified
to WLAN_REASON_MESH_CHAN_REGULATORY in the case that a regulatory
limitation was the cause for the switch. This means another station
detected a radar event.

Mark the channel as unusable if this happens.

Signed-off-by: Benjamin Berg <benjamin@sipsolutions.net>
[sw: style cleanup, rebase]
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>