cfg80211: add a helper to identify 6 GHz PSCs

This allows identifying whether or not a channel is a PSC
(preferred scanning channel).

Link: https://lore.kernel.org/r/20200528213443.414363ecf62c.Ic15e681a0e249eab7350a06ceb582cca8bb9a080@changeidSigned-off-by: Johannes Berg <johannes.berg@intel.com>

include/net/cfg80211.h

@@ -5277,6 +5277,21 @@ ieee80211_get_channel(struct wiphy *wiphy, int freq)
 	return ieee80211_get_channel_khz(wiphy, MHZ_TO_KHZ(freq));
 }
 
+/**
+ * cfg80211_channel_is_psc - Check if the channel is a 6 GHz PSC
+ * @chan: control channel to check
+ *
+ * The Preferred Scanning Channels (PSC) are defined in
+ * Draft IEEE P802.11ax/D5.0, 26.17.2.3.3
+ */
+static inline bool cfg80211_channel_is_psc(struct ieee80211_channel *chan)
+{
+	if (chan->band != NL80211_BAND_6GHZ)
+		return false;
+
+	return ieee80211_frequency_to_channel(chan->center_freq) % 16 == 5;
+}
+
 /**
  * ieee80211_get_response_rate - get basic rate for a given rate
  *