USB: zero-length read logged at Debug level (was Error, see #83)

Experience shows that some devices tend to send zero-length USB packets
even when they shouldn't, and this is a normal occurrence with these
devices.

The log level for these events has been reduced from Error to Debug in
order to minimize clutter in the log files.

Author: alexpevzner

usbtransport.go

@@ -818,7 +818,7 @@ func (conn *usbConn) Read(b []byte) (int, error) {
 		if n != 0 || err != nil {
 			return n, err
 		}
-		conn.transport.log.Error('!',
+		conn.transport.log.Debug('!',
 			"USB[%d]: zero-size read", conn.index)
 
 		time.Sleep(backoff)