USB: Add USB-ID for Multiplex RC serial adapter to cp210x.c

Hi, below patch adds the USB-ID of the serial adapters sold by
Multiplex RC (www.multiplex-rc.de).

Signed-off-by: Malte Schröder <maltesch@gmx.de>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>