USB: ftdi: support NDI devices

It enhances the driver for FTDI-based USB serial adapters to recognize and
support Northern Digital Inc (NDI) measurement equipment. NDI has been
providing this patch for various kernel flavors for several years and we would
like to see these changes built in to the driver so that our equipement works
without the need for customers to patch the kernel themselves.

The patch makes small modifications to 2 files: ./drivers/usb/serial/ftdi_sio.c
and ./drivers/usb/serial/ftdi_sio.h. It accomplishes 3 things:

1. Define the VID and PIDs to allow the driver to recognize the NDI devices.
2. Map the 19200 baud rate setting to our higher baud rate of 1.2Mb
   We would have chosen to map 38400 to the higher rate, similar to what
   several other vendors have done, but some of our legacy customers actually
   use 38400, therefore we remap 19200 to the higher rate.
3. We set the default transmit latency in the FTDI chip to 1ms for our devices.
   Our devices are typically polled at 60Hz and the default ftdi latency
   seriously affects turn-around time and results in missed data frames. We
   have created a modprobe option that allows this setting to be increased.
   This has proven necessary particularly in some virtualized environments.

Signed-off-by: Martin P. Geleynse <mgeleyns@ndigital.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>