staging: comedi: adv_pci1724: define the dac control register bits

Define the bits for the DAC control register and remove the enum along
with the helper functions.

Simplify adv_pci1724_insn_write() a bit. The 'ctrl' bits are based on
the 'chan' and 'mode' and do not need to be recalculated for each 'val'.

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>