nl80211: make sure we check for DFS with mesh channel switch

Since mesh support for DFS channels was added, we also need to check
for DFS channels when performing a channel switch with
NL80211_IFTYPE_MESHPOINT.

Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
[use switch statement, slight code cleanup]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>