staging: comedi: dt9812: use dev->class_dev for all dev_{level} messages

For consistency in the driver, use dev->class_dev for the device in
all the dev_{level} messages.

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>