staging: comedi: me_daq: tidy up dac data registers

For aesthetics, use a macro that takes the analog output channel
and returns the correct offset for the data register.

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>