mac80211: mesh: Allow following CSA to DFS channels if userspace handles it

If userspace has flagged support for DFS earlier, then we can follow CSA
to DFS channels. So instead of rejecting the switch, allow it to happen
if the flag has been set during mesh setup.

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