staging: comedi: vmk80xx: rename struct vmk80xx_usb

The struct vmk80xx_usb is actually the private data for the
comedi_device. For aesthetic reasons, rename the struct to
vmk80xx_private.

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>