wireless: Require HANDLE_DFS flag to switch channel for non-AP mode

In the case the channel should be switched to one requiring DFS we need
to make sure that userspace will handle radar events when they happen.
For AP mode this is assumed to be the case, as a manager like hostapd
is required. However IBSS and MESH modes can work without further
userspace assistance, so refuse to use DFS channels unless userspace
vouches that it handles DFS.

NOTE: Userspace should have already flagged support earlier during mesh
or IBSS setup. However, this information is not readily accessible
currently.

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