hostapd: enter DFS state if no available channel is found

Previously hostapd would not stop transmitting when a DFS event was
detected and no available channel to switch to was available.

Disable and re-enable the interface to enter DFS state. This way, TX
does not happen until the kernel notifies hostapd about the NOP
expiring.

Signed-off-by: David Bauer <mail@david-bauer.net>

package/network/services/hostapd/patches/800-dfs-enter-DFS-state-if-no-available-channel-is-found.patch

@@ -0,0 +1,37 @@
+From cefc52e6b93731c713f1bba1cb5e7e92105b758b Mon Sep 17 00:00:00 2001
+From: David Bauer <mail@david-bauer.net>
+Date: Fri, 3 Jul 2020 23:00:34 +0200
+Subject: [PATCH] dfs: enter DFS state if no available channel is found
+
+Previously hostapd would not stop transmitting when a DFS event was
+detected and no available channel to switch to was available.
+
+Disable and re-enable the interface to enter DFS state. This way, TX
+does not happen until the kernel notifies hostapd about the NOP
+expiring.
+
+Signed-off-by: David Bauer <mail@david-bauer.net>
+---
+ src/ap/dfs.c | 11 +++++++++--
+ 1 file changed, 9 insertions(+), 2 deletions(-)
+
+--- a/src/ap/dfs.c
++++ b/src/ap/dfs.c
+@@ -1066,8 +1066,15 @@ static int hostapd_dfs_start_channel_swi
+ 						  &oper_centr_freq_seg0_idx,
+ 						  &oper_centr_freq_seg1_idx,
+ 						  &skip_radar);
+-		if (!channel)
+-			return err;
++		if (!channel) {
++			/*
++			 * Toggle interface state to enter DFS state
++			 * until NOP is finished.
++			 */
++			hostapd_disable_iface(iface);
++			hostapd_enable_iface(iface);
++			return 0;
++		}
+ 		if (!skip_radar) {
+ 			iface->freq = channel->freq;
+ 			iface->conf->channel = channel->chan;