cfg80211: util: add 6GHz channel to freq conversion and vice versa

Extend the functions ieee80211_channel_to_frequency() and
ieee80211_frequency_to_channel() to support 6GHz band according
specification in 802.11ax D4.1 27.3.22.2.

Reviewed-by: Pieter-Paul Giesberts <pieter-paul.giesberts@broadcom.com>
Reviewed-by: Leon Zegers <leon.zegers@broadcom.com>
Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Link: https://lore.kernel.org/r/1564745465-21234-4-git-send-email-arend.vanspriel@broadcom.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/wireless/util.c

@@ -91,6 +91,11 @@ int ieee80211_channel_to_frequency(int chan, enum nl80211_band band)
 		else
 			return 5000 + chan * 5;
 		break;
+	case NL80211_BAND_6GHZ:
+		/* see 802.11ax D4.1 27.3.22.2 */
+		if (chan <= 253)
+			return 5940 + chan * 5;
+		break;
 	case NL80211_BAND_60GHZ:
 		if (chan < 7)
 			return 56160 + chan * 2160;
@@ -111,8 +116,11 @@ int ieee80211_frequency_to_channel(int freq)
 		return (freq - 2407) / 5;
 	else if (freq >= 4910 && freq <= 4980)
 		return (freq - 4000) / 5;
-	else if (freq <= 45000) /* DMG band lower limit */
+	else if (freq < 5940)
 		return (freq - 5000) / 5;
+	else if (freq <= 45000) /* DMG band lower limit */
+		/* see 802.11ax D4.1 27.3.22.2 */
+		return (freq - 5940) / 5;
 	else if (freq >= 58320 && freq <= 70200)
 		return (freq - 56160) / 2160;
 	else