Sometimes controller was replying with the Data Error 5 message.
  Logic analyzer studies have shown that this occues when some
delay between the control message bytes appears.
  The only reason (except internal hardware problems) was tcdrain()
before each control message byte sending. In this commit tcdrain()
(which waits until the transmit buffer is empty) is placed only
before the first control message byte. And it seems that no more
Data Error 5 is seen.