tcpip: drop OP_FLAG_TERMCHAR_SET from device_write
tcpip: drop OP_FLAG_TERMCHAR_SET from device_write

Problem: ICS 8065 ethernet-GPIB gateway returns a failure
to the device_write RPC in a simple *idn? query to a
gpib-connected instrument.

TCPIPInstrSession.write() sets the OP_FLAG_TERMCHAR_SET flag,
but the VXI-11 spec says:

  termchrset(80): This flag is set to one(1) if a termination character
  is specified on a read. The actual termination character itself is
  passed in the termchr parameter. This flag is only valid for device_read.

Solution: never set this flag on device_write.

Fixes #225

hgrecco/pyvisa-py,pyvisa/pyvisa-py