staging: comedi: dt282x: use dev->write_subdev

Use the dev->write_subdev that was setup in the device attach instead
of accessing the dev->subdevices array directly.

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>