USB: serial: option.c: use module_usb_serial_driver

This converts the option.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: Matthias Urlichs <smurf@smurf.noris.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>