staging: comedi: adv_pci1723: remove private data 'da_range'

This member of the private data is initialized to 0 by pci1723_reset()
and is not used any where else in the driver. Remove it.

Use the register map defines to set the range for each output channel.

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>