iio: dac: cio-dac: Cleanup indexing for DAC writes

Simplify DAC write code by defining base member as u16 __iomem *; DAC
registers are 16-bit so this allows us to index each DAC channel
directly in a loop rather than calculating the offsets by multipling by
2 each time.

Signed-off-by: William Breathitt Gray <william.gray@linaro.org>
Link: https://lore.kernel.org/r/d9dab6696af7eabb2d46f5cbc7871329f499c1c9.1657213745.git.william.gray@linaro.org


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>