autotest: fix FRSkyPassThroughSensorIDs test

need to clear the read buffers before re-polling sensors - just in case we don't need to!

Tools/autotest/vehicle_test_suite.py

@@ -1715,8 +1715,8 @@ def update(self):
             if not self.connect():
                 self.progress("Failed to connect")
                 return
-        self.check_poll()
         self.do_sport_read()
+        self.check_poll()
 
     def do_sport_read(self):
         self.buffer += self.do_read()