USB: serial: cp210x: added Ketra N1 wireless interface support

Added support for Ketra N1 wireless interface, which uses the
Silicon Labs' CP2104 USB to UART bridge with customized PID 8946.

Signed-off-by: Joe Savage <joe.savage@goketra.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Johan Hovold <johan@kernel.org>