iio: accel: kxsd9: Do away with the write2 helper

This is just a masquerading register write function, so use the
register write function instead.

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