staging: comedi: vmk80xx: sanity check context used to get the boardinfo

As done in all the comedi drivers that auto attach, sanity check the
passed context that is used to get the boardinfo.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>