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

The usb_driver (*disconnect) calls comedi_usb_auto_unconfig() which will
call the comedi_driver (*detach). Just move all the disconnect/detach
into the comedi_driver and use 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>