staging: comedi: convert class code to use dev_groups

The dev_attrs field of struct class is going away soon, dev_groups
should be used instead.  This converts the comedi class code to use the
correct field.

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