staging: comedi: vmk80xx: use comedi_to_usb_dev()

Use the helper function to get a usb_device pointer from a
comedi_device pointer. This removes the need carring the
usb_device pointer in the private data.

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>