cfg80211: fix dfs channels remain DFS_AVAILABLE after ch_switch

Depending on the regulatory domain, leaving a DFS channel requires
a new CAC to be performed when returning back to that channel.
If needed, update dfs states after a driver channel switch.

Signed-off-by: Michael Vassernis <michael.vassernis@tandemg.com>
Link: https://lore.kernel.org/r/20190729060024.5660-1-michael.vassernis@tandemg.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>