iio: accel: kxsd9: Deploy proper register bit defines

There are some hardcoded register values etc in the code, define
proper bitfield definitions, and use them when getting and setting
the scale. Optimize a read/modify/write to use regmap_update_bits()
at the same time.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Tested-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>