wifi: nl80211: expose can-monitor channel property

It may be possible to monitor on disabled channels per the
can-monitor flag, but evidently I forgot to expose that out
to userspace. Fix that.

Fixes: a110a3b7 ("wifi: cfg80211: optionally support monitor on disabled channels")
Reviewed-by: Miriam Rachel Korenblit <miriam.rachel.korenblit@intel.com>
Reviewed-by: Ilan Peer <ilan.peer@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Link: https://msgid.link/20240523120945.9a2c19a51e53.I50fa1b1a18b70f63a5095131ac23dc2e71f3d426@changeidSigned-off-by: Johannes Berg <johannes.berg@intel.com>

net/wireless/nl80211.c

@@ -1204,6 +1204,9 @@ static int nl80211_msg_put_channel(struct sk_buff *msg, struct wiphy *wiphy,
 		if ((chan->flags & IEEE80211_CHAN_NO_6GHZ_AFC_CLIENT) &&
 		    nla_put_flag(msg, NL80211_FREQUENCY_ATTR_NO_6GHZ_AFC_CLIENT))
 			goto nla_put_failure;
+		if ((chan->flags & IEEE80211_CHAN_CAN_MONITOR) &&
+		    nla_put_flag(msg, NL80211_FREQUENCY_ATTR_CAN_MONITOR))
+			goto nla_put_failure;
 	}
 
 	if (nla_put_u32(msg, NL80211_FREQUENCY_ATTR_MAX_TX_POWER,