staging: comedi: dt9812: Resolve different base types warnings.

Resolves warnings from the "sparse" checker of the form "warning: incorrect
type in assignment (different base types)".

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