USB: serial: ftdi_sio: add device IDs for Sienna and Echelon PL-20

Both devices added here have a FTDI chip inside. The device from Echelon
is called 'Network Interface' it is actually a LON network gateway.

 ID 0403:8348 Future Technology Devices International, Ltd
 https://www.eltako.com/fileadmin/downloads/de/datenblatt/Datenblatt_PL-SW-PROF.pdf

 ID 0920:7500 Network Interface
 https://www.echelon.com/products/u20-usb-network-interface

Signed-off-by: Beni Mahler <beni.mahler@gmx.net>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Johan Hovold <johan@kernel.org>