staging: comedi: vmk80xx: use comedi_auto_unconfig() for (*disconnect)

The usb_driver (*disconnect) in this driver is simply a wrapper around
comedi_auto_unconfig(). Just use comedi_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>