staging: comedi: usbduxfast: Use comedi_usb_auto_unconfig() for (*disconnect)

The usb_driver (*disconnect) calls comedi_usb_auto_unconfig() then
frees any allocations by calling tidy_up(). comedi_usb_auto_unconfig()
calls the comedi_driver (*detach) function, usbduxfast_detach().

Move the tidy_up() call into usbduxfast_detach() and use the
comedi_usb_auto_unconfig() directly for the (*disconnect).

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>