cfg80211: allow CFG80211_SIGNAL_TYPE_UNSPEC in station_info

The station_info struct had demanded dBm signal values, but the
cfg80211 wireless extensions implementation was also accepting
"unspecified" (i.e. RSSI) unit values while the nl80211 code was
completely unaware of them.  Resolve this by formally allowing the
"unspecified" units while making nl80211 ignore them.

Signed-off-by: John W. Linville <linville@tuxdriver.com>
Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
diff --git a/include/net/cfg80211.h b/include/net/cfg80211.h
index 69b7ad3..5ccac72 100644
--- a/include/net/cfg80211.h
+++ b/include/net/cfg80211.h
@@ -619,8 +619,10 @@
  * @llid: mesh local link id
  * @plid: mesh peer link id
  * @plink_state: mesh peer link state
- * @signal: signal strength of last received packet in dBm
- * @signal_avg: signal strength average in dBm
+ * @signal: the signal strength, type depends on the wiphy's signal_type
+	NOTE: For CFG80211_SIGNAL_TYPE_MBM, value is expressed in _dBm_.
+ * @signal_avg: avg signal strength, type depends on the wiphy's signal_type
+	NOTE: For CFG80211_SIGNAL_TYPE_MBM, value is expressed in _dBm_.
  * @txrate: current unicast bitrate from this station
  * @rxrate: current unicast bitrate to this station
  * @rx_packets: packets received from this station