staging: comedi: dt282x: fix bug where DIO direction is reset to inputs

The direction of the 16 DIO channels is controlled with two bits in the
DACSR register. The HBOE bit controls the direction of channels 15-8 and
the LBOE bit channels 7-0.

The DACSR register is also used to control the Analog Output async command
support. Currently, starting or canceling an Analog Output command would
also reset the DIO directions so that all channels were inputs.

Fix the driver so that the DIO direction is not changed when starting or
canceling an Analog Output async command.

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>