USB: serial: belkin_sa.c: use module_usb_serial_driver

This converts the belkin_sa.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: William Greathouse <wgreathouse@smva.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>