staging: comedi: usbduxsigma: refactor init code

Refactor the usb driver probe/disconnect and comedi driver attach/
detach to follow the style of the other comedi driver types to
improve maintainability.
Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Cc: Frank Mori Hess <fmhess@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>