b43: fix frequency reported on G-PHY with /new/ firmware

Support for firmware rev 508+ was added years ago, but we never noticed
it reports channel in a different way for G-PHY devices. Instead of
offset from 2400 MHz it simply passes channel id (AKA hw_value).

So far it was (most probably) affecting monitor mode users only, but
the following recent commit made it noticeable for quite everybody:

commit 3afc2167
Author: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Date:   Tue Mar 4 16:50:13 2014 +0200

    cfg80211/mac80211: ignore signal if the frame was heard on wrong channel
Reported-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Cc: stable@vger.kernel.org
Tested-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/b43/xmit.c

@@ -811,8 +811,12 @@ void b43_rx(struct b43_wldev *dev, struct sk_buff *skb, const void *_rxhdr)
 		break;
 	case B43_PHYTYPE_G:
 		status.band = IEEE80211_BAND_2GHZ;
-		/* chanid is the radio channel cookie value as used
-		 * to tune the radio. */
+		/* Somewhere between 478.104 and 508.1084 firmware for G-PHY
+		 * has been modified to be compatible with N-PHY and others.
+		 */
+		if (dev->fw.rev >= 508)
+			status.freq = ieee80211_channel_to_frequency(chanid, status.band);
+		else
 			status.freq = chanid + 2400;
 		break;
 	case B43_PHYTYPE_N: