USB: serial: cp210x.c: use module_usb_serial_driver

This converts the cp210x.c driver to use the module_usb_serial_driver() call
instead of having to have a module_init/module_exit function, saving a lot
of duplicated code.

CC: Johan Hovold <jhovold@gmail.com>
CC: Rusty Russell <rusty@rustcorp.com.au>
CC: Florian Fainelli <f.fainelli@gmail.com>
CC: "Malte Schröder" <maltesch@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>