Fix I2C peripheral scanning

Sending STOP immediately after a READ address is illegal and puts the
peripheral in an undefined state.

Author: bessman

pslab/bus/i2c.py

@@ -515,7 +515,7 @@ def ping(self) -> bool:
         response : bool
             True is slave responded, False otherwise.
         """
-        response = self._start(self.address, self._READ)
+        response = self._start(self.address, self._WRITE)
         self._stop()
 
         return response == self._ACK