USB: serial: pl2303: add device id for ztek device

This adds a new device id to the pl2303 driver for the ZTEK device.

Reported-by: Mike Chu <Mike-Chu@prolific.com.tw>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>