staging: comedi: me_daq: tidy up dac control register defines

Redefine the dac control register bits as macros that take the
channel number and return the correct bit values.

This register needs to be read after the new values are written
in order to actually update the dac. For aesthetics, use the same
define to read and write the registers.

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>