nl80211: Disallow setting of HT for channel 14

This patch disables setting of HT20 and more for channel 14 because
the channel is only for IEEE 802.11b.

The patch for net/wireless/util.c was unit-tested.

The patch for net/wireless/chan.c was tested with iw command.

Before this patch.
$ sudo iw dev <ifname> set channel 14 HT20
$

After this patch.
$ sudo iw dev <ifname> set channel 14 HT20
kernel reports: invalid channel definition
command failed: Invalid argument (-22)
$
Signed-off-by: Masashi Honma <masashi.honma@gmail.com>
Link: https://lore.kernel.org/r/20191021075045.2719-1-masashi.honma@gmail.com
[clean up the code, use != instead of equivalent >]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/wireless/chan.c

@@ -204,6 +204,11 @@ bool cfg80211_chandef_valid(const struct cfg80211_chan_def *chandef)
 		return false;
 	}
 
+	/* channel 14 is only for IEEE 802.11b */
+	if (chandef->center_freq1 == 2484 &&
+	    chandef->width != NL80211_CHAN_WIDTH_20_NOHT)
+		return false;
+
 	if (cfg80211_chandef_is_edmg(chandef) &&
 	    !cfg80211_edmg_chandef_valid(chandef))
 		return false;

net/wireless/util.c

@@ -1559,7 +1559,8 @@ bool ieee80211_chandef_to_operating_class(struct cfg80211_chan_def *chandef,
 	}
 
 	if (freq == 2484) {
-		if (chandef->width > NL80211_CHAN_WIDTH_40)
+		/* channel 14 is only for IEEE 802.11b */
+		if (chandef->width != NL80211_CHAN_WIDTH_20_NOHT)
 			return false;
 
 		*op_class = 82; /* channel 14 */