Copter: change default streamrate for ADSB from 5 to 0

No other stream has a default streamrate in Copter.

This causes us small amounts of overhead as it marks all channels as
streaming.

ArduCopter/GCS_Mavlink.cpp

@@ -444,7 +444,7 @@ const AP_Param::GroupInfo GCS_MAVLINK::var_info[] = {
     // @Range: 0 50
     // @Increment: 1
     // @User: Advanced
-    AP_GROUPINFO("ADSB",   9, GCS_MAVLINK, streamRates[9],  5),
+    AP_GROUPINFO("ADSB",   9, GCS_MAVLINK, streamRates[9],  0),
 AP_GROUPEND
 };