USB: serial: digi_acceleport.c: use module_usb_serial_driver

This converts the digi_acceleport.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: Peter Berger <pberger@brimson.com>
CC: Al Borchers <alborchers@steinerpoint.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>