staging: comedi: das08: add DAS08JR_AO_UPDATE_REG

staging: comedi: das08: add DAS08JR_AO_UPDATE_REG

"JR" boards with analog output channels have a jumper that controls
whether analog output channels are updated simultaneously or
individually.  When set to update individually, individual channels are
updated when the high byte register is written. When set to update
simultaneously, channels are not updated until the digital inputs
register is read.  The driver doesn't know how the jumper is set and is
not interested in the simultaneous output feature, so it updates a
channel by writing the low byte, then the high byte, then reading the
digital inputs register.  To make the code more explicit, add a macro
`DAS08JR_AO_UPDATE_REG` with the same value as the `DAS08JR_DI_REG`
macro (for digital inputs) and use it when reading the register to
update the analog outputs.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>