airspeed: raise retries to 2

we're still getting occasional airspeed dropouts with it at 1, even
with reasonable cabling. Raising it to 2 should fix this.

Author: Andrew Tridgell

src/drivers/airspeed/airspeed.cpp

@@ -166,8 +166,8 @@ Airspeed::probe()
 	_retries = 4;
 	int ret = measure();
 
-        // drop back to 1 retry once initialised
-	_retries = 1;
+        // drop back to 2 retries once initialised
+	_retries = 2;
 	return ret;
 }