iio: accel: kxsd9: Convert to use regmap for transport

This converts the KXSD9 driver to drop the custom transport
mechanism and just use regmap like everything else.

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