staging: comedi: usbduxsigma: push usb (*disconnect) into comedi (*detach)

The usb_driver (*disconnect) already calls comedi_usb_auto_unconfig(), which
will call the comedi_driver (*detach). Move the other operations in the
(*disconnect) into the (*detach). The comedi_usb_auto_unconfig() can then
be used 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>