staging: comedi: vmk80xx: move usb_driver (*disconnect) code

The usb_driver (*disconnect) in this driver calls the comedi core
comedi_usb_auto_unconfig() which calls the comedi_driver (*detach).

Move the code in the (*disconnect) to the (*detach) to get all the
disconnect/detach in one place.

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>