APM: change WIND direction to match convention

Thanks to Leo Hogg for pointing this out

ArduPlane/GCS_Mavlink.pde

@@ -455,7 +455,8 @@ static void NOINLINE send_wind(mavlink_channel_t chan)
     Vector3f wind = ahrs.wind_estimate();
     mavlink_msg_wind_send(
         chan,
-        degrees(atan2(wind.y, wind.x)),
+        degrees(atan2(-wind.y, -wind.x)), // use negative, to give
+                                          // direction wind is coming from
         sqrt(sq(wind.x)+sq(wind.y)),
         wind.z);
 }

libraries/GCS_MAVLink/message_definitions/ardupilotmega.xml

@@ -306,7 +306,7 @@
 
 	  <message name="WIND" id="168">
 	    <description>Wind estimation</description>
-            <field type="float" name="direction">wind direction (degrees)</field>
+            <field type="float" name="direction">wind direction that wind is coming from (degrees)</field>
             <field type="float" name="speed">wind speed in ground plane (m/s)</field>
             <field type="float" name="speed_z">vertical wind speed (m/s)</field>
 	  </message>