staging: comedi: vmk80xx: remove need for boardinfo in private_data

The only information in the boardinfo that is used outside of the
attach of the driver is the 'model' of the device.

Remove the 'board' pointer from the private data and replace it with
the 'model' enum and just copy that information over during the attach.

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>